 This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer, please visit LibriVox.org. Recorded by Peter Yersley. The Origin of Species by Means of Natural Selection or The Preservation of Favoured Races in the Struggle for Life 6. London Edition by Charles Darwin Chapter No. 14 Section 1 Mutual Affinities of Organic Beings Morphology, Embryology, Rudimentary Organs Contents of this chapter include Classification, Groups Subordinate to Groups Natural System, Rules and Difficulties in Classification Explained on the Theory of Descent with Modification Classification of Varieties Descent always used in Classification Analogical or Adaptive Characters Affinities, General, Complex and Radiating Extinction separates and defines groups Morphology between members of the same class between parts of the same individual Embryology, Laws of Explained by Variations Not Supervening at an Early Age and Being Inherited at a Corresponding Age Rudimentary Organs, Their Origin Explained Summary Classification From the most remote period in the history of the world organic beings have been found to resemble each other in descending degrees so that they can be classed in groups under groups This classification is not arbitrary like the grouping of the stars in constellations The existence of groups would have been of simple significance if one group had been exclusively fitted to inhabit the land and another the water one to feed on flesh, another on vegetable matter and so on but the case is widely different for it is notorious how commonly members of even the same subgroup have different habits In the second and fourth chapters on variation and on natural selection I have attempted to show that within each country it is the widely ranging, the much diffused and common that is the dominant species belonging to the larger genera in each class which vary most The varieties, or incipient species thus produced ultimately become converted into new and distinct species and these, on the principle of inheritance tend to produce other new and dominant species consequently the groups which are now large and which generally include many dominant species tend to go on increasing in size I further attempted to show that from the varying descendants of each species trying to occupy as many and as different places as possible in the economy of nature they constantly tend to diverge in character This latter conclusion is supported by observing the great diversity of forms which in any small area come into the closest competition and by certain facts in naturalization I attempted also to show that there is a steady tendency in the forms which are increasing in number and diverging in character to supplant and exterminate the preceding less divergent and less improved forms I request the reader to turn to the diagram illustrating the action as formally explained of these several principles and he will see that the inevitable result is that the modified descendants proceeding from one progenitor become broken up into groups subordinate to groups In the diagram each letter on the uppermost line may represent a genus including several species and the whole of the genera along this upper line form together one class for all are descended from one ancient parent and consequently have inherited something in common but the three genera on the left hand have on this same principle much in common and form a subfamily distinct from that containing the next two genera on the right hand which diverged from a common parent at the fifth stage of descent These five genera have also much in common though less than when grouped in subfamilies and they form a family distinct from that containing the three genera still further to the right hand which diverged at an earlier period and all these genera descended from A form an order distinct from the genera descended from I so that we here have many species descended from a single progenitor grouped into genera and the genera into subfamilies, families and orders all under one great class The grand fact of the natural subordination of organic beings in groups under groups which from its familiarity does not always sufficiently strike us is in my judgment thus explained no doubt organic beings like all other objects can be classed in many ways either artificially by single characters or more naturally by a number of characters we know for instance that minerals and the elemental substances can thus be arranged in this case there is of course no relation to genealogical succession and no cause can at present be assigned for their falling into groups but with organic beings the case is different and the view above given accords with a natural arrangement in group under group and no other explanation has ever been attempted naturalists as we have seen try to arrange the species genera and families in each class on what is called the natural system but what is meant by this system some authors look at it merely as a scheme for arranging together those living objects which are most alike and for separating those which are most unlike or as an artificial method of enunciating as briefly as possible general propositions that is by one sentence to give the characters common for instance to all mammals by another those common to all carnivora by another those common to the dog genus and then by adding a single sentence a full description is given of each kind of dog the ingenuity and utility of this system are indisputable but many naturalists think that something more is meant by the natural system they believe that it reveals the plan of the creator but unless it be specified whether order in time or space or both or what else is meant by the plan of the creator it seems to me that nothing is thus added to our knowledge expressions such as that famous one by Linnaeus which we often meet with in a more or less concealed form namely that the characters do not make the genus but that the genus gives the characters seem to imply that some deeper bond is included in our classifications than mere resemblance I believe that this is the case and that community of descent the one known cause of close similarity in organic beings is the bond which though observed by various degrees of modification is partly revealed to us by our classifications let us now consider the rules followed in classification and the difficulties which are encountered on the view that classification either gives some unknown plan of creation or is simply a scheme for enunciating general propositions and of placing together the forms most like each other it might have been thought and was in ancient times thought that those parts of the structure which determined the habits of life and the general place of each being in the economy of nature would be of very high importance in classification nothing can be more false no one regards the external similarity of a mouse to a shrew of a dugong to a whale of a whale to a fish as of any importance these resemblances though so intimately connected with the whole life of the being are ranked as merely adaptive or analogical characters but to the consideration of these resemblances we shall recur it may even be given as a general rule that the less any part of the organisation is concerned with special habits the more important it becomes for classification as an instance Owen in speaking of the dugong says the generative organs being those which are most remotely related to the habits and food of an animal I have always regarded as affording very clear indications of its true affinities we are least likely in the modifications of these organs to mistake a merely adaptive for an essential character with plants how remarkable it is that the organs of vegetation on which the nutrition and life depend are of little signification whereas the organs of reproduction with their product the seed and the embryo are of paramount importance so again informally discussing certain morphological characters which are not functionally important we have seen that they are often of the highest service in classification this depends on their constancy throughout many allied groups and their constancy chiefly depends on any slight deviations not having been preserved and accumulated by natural selection which acts only on serviceable characters that the mere physiological importance of an organ does not determine its classificatory value is almost proved by the fact that in allied groups in which the same organ as we have every reason to suppose has nearly the same physiological value its classificatory value is widely different no naturalist can have worked at any group without being struck with this fact and it has been fully acknowledged in the writings of almost every author it will suffice to quote the highest authority Robert Brown who in speaking of certain organs in the protease says their generic importance like that of all their parts, not only in this but as I apprehend in every natural family is very unequal and in some cases seems to be entirely lost again in another work he says the genera of the connoissee differ in having one or more ovaria in the existence or absence of albumin in the imbracate or valvular east evasion any one of these characters singly is frequently of more than generic importance though here even when taken all together they appear insufficient to separate nestis from canaris to give an example among insects in one great division of the hymenoptera the antennae as westwood has remarked are most constant in structure in another division they differ much and the differences are of quite subordinate value in classification yet no one will say that the antennae in these two divisions of the same order are of unequal physiological importance any number of instances could be given of the varying importance for classification of the same important organ within the same group of beings again no one will say that rudimentary or atrophied organs are of high physiological or vital importance yet undoubtedly organs in this condition are often of much value in classification no one will dispute that the rudimentary teeth in the upper jaws of young ruminants and certain rudimentary bones of the leg are highly serviceable in exhibiting the close affinity between ruminants and pachyderms Robert Brown has strongly insisted on the fact that the position of the rudimentary florets is of the highest importance in the classification of the grasses numerous instances could be given of characters derived from parts which must be considered of very trifling physiological importance but which are universally admitted as highly serviceable in the definition of whole groups for instance whether or not there is an open passage from the nostrils to the mouth the only character according to Owen which absolutely distinguishes fishes and reptiles the inflection of the angle of the lower jaw in marsupials the manner in which the wings of insects are folded mere colour in certain algae mere pubescence on parts of the flower in grasses the nature of the dermal covering as hair or feathers in the vertebrata if the ornithorhynchus had been covered with feathers instead of hair this external and trifling character would have been considered by naturalists as an important aid in determining the degree of affinity of this strange creature to birds the importance for classification of trifling characters mainly depends on their being correlated with many other characters of more or less importance the value indeed of an aggregate of characters is very evident in natural history hence as has often been remarked a species may depart from its allies in several characters both of high physiological importance and of almost universal prevalence and yet leave us in no doubt where it should be ranked hence also it has been found that a classification founded on any single character however important that may be has always failed for no part of the organisation is invariably constant the importance of an aggregate of characters even when none are important alone explains the aphorism enunciated by Linnaeus namely that the characters do not give the genus but the genus gives the character for this seems founded on the appreciation of many trifling points of resemblance too slight to be defined certain plants belonging to the Malpighiaceae bear perfect and degraded flowers in the latter as ad-déjuicea has remarked the greater number of the characters proper to the species to the genus to the family to the class disappear and thus laugh at our classification when Aspicapa produced in France during several years only those degraded flowers departing so wonderfully in a number of the most important points of structure on the proper type of the order yet Monsieur Grichard sur-gaciously saw as Jusiae observes that this genus should still be retained among the Malpighiaceae this case well illustrates the spirit of our classifications practically when naturalists are at work they do not trouble themselves about the physiological value of the characters which they use in defining a group or in allocating any particular species if they find a character nearly uniform and common to a great number of forms and not common to others they use it as one of high value if common to some lesser number they use it as of subordinate value this principle has been broadly confessed by some naturalists to be the true one and by none more clearly than by that excellent botanist Auguste Saint-Hilaire if several trifling characters are always found in combination there no apparent bond of connection can be discovered between them a special value is set on them as in most groups of animals important organs such as those for propelling the blood or for aerating it or those for propagating the race are found nearly uniform they are considered as highly serviceable in classification but in some groups all these the most important vital organs are found to offer characters of quite subordinate value thus as Fritz Müller has lately remarked in the same group of crustaceans chypridina is furnished with a heart while in two closely allied genera namely chypris and chytheria there is no such organ one species of chypridina has well developed branchii while another species is destitute of them we can see why characters derived from the embryo should be of equal importance with those derived from the adult for a natural classification of course includes all ages but it is by no means obvious on the ordinary view why the structure of the embryo should be more important for this purpose than that of the adult which alone plays its full part in the economy of nature yet it has been strongly urged by those great naturalists Milne Edwards and Agassiz that embryological characters are the most important of all and this doctrine has very generally been admitted as true nevertheless their importance has sometimes been exaggerated going to the adaptive characters of larvae not having been excluded in order to show this Fritz Muller arranged by the aid of such characters alone the great class of crustaceans and the arrangements did not prove a natural one but there can be no doubt that embryonic excluding larval characters are of the highest value for classification not only with animals but with plants thus the main divisions of flowering plants are founded on differences in the embryo on the number and position of the cotyledons and on the mode of development of the plumeule and radical we shall immediately see why these characters possess so higher value in classification namely from the natural system being genealogical in its arrangement our classifications are often plainly influenced by chains of affinities nothing can be easier than to define a number of characters common to all birds but with crustaceans any such definition has hitherto been found impossible there are crustaceans at the opposite ends of the series which have hardly a character in common yet the species at both ends from being plainly allied to others and these to others and so onwards can be recognized as unequivocally belonging to this and to no other class of the articulata geographical distribution has often been used though perhaps not quite logically in classification more especially in very large groups of closely allied forms temmink insists on the utility or even a necessity of this practice in certain groups of birds and it has been followed by several entomologists and botanists finally with respect to the comparative value of the various groups of species such as orders, sub-orders, families, sub-families and genera they seem to be at least at present almost arbitrary several of the best botanists such as Mr. Bentham and others have strongly insisted on their arbitrary value instances could be given among plants and insects of a group first ranked by practised naturalists as only a genus then raised to the rank of a sub-family or family and this has been done not because further research has detected important structural differences at first overlooked but because numerous allied species with slightly different grades of difference have been subsequently discovered all the foregoing rules and aids and difficulties in classification may be explained if I do not greatly deceive myself on the view that the natural system is founded on dissent with modification that the characters which naturalists consider as showing true affinity between any two or more species are those which have been inherited from a common parent all true classification being genealogical that community of dissent is the hidden bond which naturalists have been unconsciously seeking and not some un-known plan of creation or the enunciation of general propositions and the mere putting together and separating objects more or less alike but I must explain my meaning more fully I believe that the arrangement of the groups within each class in due sub-ordination and relation to each other must be strictly genealogical in order to be natural but that's the amount of difference in the several branches or groups though allied in the same degree in blood to their common progenitor may differ greatly being due to the different degrees of modification which they have undergone and this is expressed by the forms being ranked under different genera, families, sections or orders the reader will best understand what is meant if he will take the trouble to refer to the diagram in the fourth chapter we will suppose the letters A to L to represent allied genera existing during the Silurian epoch and descended from some still earlier form in three of these genera, A, F and I a species has transmitted modified descendants to the present day represented by the 15 genera A14 to Z14 on the uppermost horizontal line now all these modified descendants from a single species are related in blood or descent in the same degree they may metaphorically be called cousins to the same millionth degree yet they differ widely and in different degrees from each other the forms descended from A now broken up into two or three families constitute a distinct order from those descended from I also broken up into two families nor can the existing species descended from A be ranked in the same genus with the parent A or those from I with parent I but the existing genus F14 may be supposed to have been but slightly modified and it will then rank with the parent genus F just as some few still living organisms belong to Silurian genera so that the comparative value of the difference between these organic beings which are all related to each other in the same degree in blood has come to be widely different nevertheless their genealogical arrangement remains strictly true not only at the present time but at each successive period of descent all the modified descendants from A will have inherited something in common from their common parent as will all the descendants from I so will it be with each subordinate branch of descendants at each successive stage if however we suppose any descendant of A or of I to have become so much modified as to have lost all traces of its parentage in this case its place in the natural system will be lost as seems to have occurred with some few existing organisms all the descendants of the genus F along its whole line of descent are supposed to have been but little modified and they form a single genus as this genus though much isolated will still occupy its proper intermediate position the representation of the groups as here given in the diagram on a flat surface is much too simple the branches ought to have diverged in all directions if the names of the groups had been simply written down in a linear series the representation would have been still less natural which is notoriously not possible to represent in a series on a flat surface the affinities which we discover in nature among the beings of the same group thus the natural system is genealogical in its arrangement like a pedigree but the amount of modification which the different groups have undergone has to be expressed by ranking them under different so called sub-families, families, sections, orders and classes it may be worthwhile to illustrate this view of classification by taking the case of languages if we possessed a perfect pedigree of mankind a genealogical arrangement of the races of man would afford the best classification of the various languages now spoken throughout the world and if all extinct languages and all intermediate and slowly changing dialects were to be included such an arrangement would be the only possible one yet it might be that some ancient languages had altered very little and had given rise to few new languages while others had altered much owing to the spreading, isolation and state of civilization of the several co-descended races and had thus given rise to many new dialects and languages the various degrees of difference between the languages of the same stock would have to be expressed by groups subordinate to groups but the proper or even the only possible arrangement would still be genealogical and this would be strictly natural as it would connect together all the languages extinct and recent by the closest affinities and would give the filiation and the origin of each tongue in confirmation of this view let us glance at the classification of varieties which are known or believed to be descended from a single species these are grouped under the species with the sub-varieties under the varieties and in some cases as with the domestic pigeon with several other grades of difference nearly the same rules are followed as in classifying species authors have insisted on the necessity of arranging varieties on a natural instead of an artificial system we are cautioned for instance not to class two varieties of the pineapple together merely because their fruit though the most important part happens to be nearly identical no one puts the Swedish and common turnic together though the escalant and thickened stems are so similar whatever part is found to be most constant is used in classing varieties thus the great agriculturist Marshall says the horns are very useful for this purpose with cattle because they are less variable than the shape or color of the body etc whereas with sheep the horns are much less serviceable because less constant in classing varieties I apprehend that if we had a real pedigree a genealogical classification would be universally preferred and it has been attempted in some cases for we might feel sure whether there had been more or less modification that the principle of inheritance would keep the forms together which were allied in the greatest number of points in tumbler pigeons though some of the sub varieties differ in the important character of the length of the beak yet all are kept together from having the common habit of tumbling but the short faced breed has nearly or quite lost this habit nevertheless without any thought on the subject these tumblers are kept in the same group because allied in blood and alike in some other respects with species in a state of nature every naturalist has in fact brought descent into his classification for he includes in his lowest grade that of species the two sexes and how enormously these some differ in the most important characters is known to every naturalist scarcely a single fact can be predicated in common of the adult males and hermaphrodites of certain syrupses and yet no one dreams of separating them as soon as the three orchidian forms monocanthus myanthus and catacetam which had previously been ranked as three distinct genera were known to be sometimes produced on the same plant they were immediately considered as varieties and now I have been able to show that they are the male female and hermaphrodite forms of the same species the naturalist includes as one species the various larval stages of the same individual however much they may differ from each other and from the adult as well as the so called alternate generations of steenstrupp not only in a technical sense be considered as the same individual he includes monsters and varieties not from their partial resemblance to the parent form but because they are descended from it as descent has universally been used in classing together the individuals of the same species though the males and females and larvae are sometimes extremely different and as it has been used in classing varieties which have undergone a certain and sometimes a considerable amount of modification may not this same element of descent have been unconsciously used in grouping species under genera and genera under higher groups all under the so called natural system I believe it has been unconsciously used and thus only can I understand the several rules and guides which have been followed by our best systematists as we have no written pedigrees we are forced to trace community of descent by resemblances of any kind therefore we choose those characters which are the least likely to have been modified in relation to the conditions of life to which each species has been recently exposed rudimentary structures on this view are as good as or even sometimes better than other parts of the organization we care not how trifling a character may be let it be the mere inflection of the angle of the jaw the manner in which an insect's wing is folded whether the skin be covered by hair or feathers if it prevails throughout many and different species especially those having very different habits of life it assumes high value for we can account for its presence in so many forms with such different habits only by inheritance from a common parent we may err in this respect in regard to single points of structure but when several characters let them be ever so trifling concur throughout a large group of beings having different habits we may feel almost sure on the theory of descent that these characters have been inherited from a common ancestor and we know that such aggregated characters have a special value in classification we can understand why a species or a group of species may depart from its allies in several of its most important characteristics and yet be safely classed with them this may be safely done and is often done as long as a sufficient number of characters let them be ever so unimportant betrays the hidden bond of community of descent let two forms have not a single character in common yet if these extreme forms are connected together by a chain of intermediate groups we may at once infer their community of descent and we put them all into the same class as we find organs of high physiological importance those which serve to preserve life under the most diverse conditions of existence are generally the most constant we attach a special value to them but if these same organs in another group or section of a group are found to differ much we at once value them less in our classification we shall presently see why embryological characters are of such high classificatory importance geographical distribution may sometimes be brought usefully into play in classing large genera because all the species of the same genus inhabiting any distinct and isolated region are in all probability descended from the same parents analogical resemblances we can understand on the above views the very important distinction between real affinities and analogical or adaptive resemblances Lamarck first called attention to this subject and he has been ably followed by McClee and others the resemblance in the shape of the body and in the fin-like anterior limbs between dugongs and whales and between these two orders of mammals and fishes are analogical so is the resemblance between a mouse and a shrew mouse, Sorex which belong to different orders and the still closer resemblance insisted on by Mr. Mivart between the mouse and a small marsupial animal Antikynus of Australia these latter resemblances may be accounted for as it seems to me by adaptation for similarly active movements through thickets and herbage together with concealment from enemies among insects there are innumerable instances thus Linnaeus, misled by external appearances actually classed an homopterous insect as a moth we see something of the same kind even with our domestic varieties as in the strikingly similar shape of the body in the improved breeds of the Chinese and common pig which are descended from distinct species and in the similarly thickened stems of the common and specifically distinct Swedish turnip the resemblance between the greyhound and racehorse is hardly more fanciful than the analogies which have been drawn by some authors between widely different animals on the view of characters being of real importance and classification only insofar as they reveal descent we can clearly understand why analogical or adaptive characters although of the utmost importance to the welfare of the being are almost valueless to the systematist for animals belonging to two most distinct lines of descent may have become adapted to similar conditions and thus have assumed a close external resemblance but such resemblances will not reveal whether tends to conceal their blood relationship we can thus also understand the apparent paradox that the very same characters are analogical when one group is compared with another but give true affinities when the members of the same group are compared together thus the shape of the body and fin-like limbs are only analogical when whales are compared with fishes being adaptations in both classes for swimming through the water but between the several members of the whale family the shape of the body and the fin-like limbs offer characters exhibiting true affinity for as these parts are so nearly similar throughout the whole family we cannot doubt that they have been inherited from a common ancestor so it is with fishes numerous cases could be given of striking resemblances in quite distinct beings between single parts or organs adapted for the same functions a good instance is afforded by the close resemblance of the jaws of the dog and Tasmanian wolf or Thylassinus, animals which are widely sundered in the natural system but this resemblance is confined to general appearance as in the prominence of the canines and in the cutting shape of the molar teeth for the teeth really differ much thus the dog has on each side of the upper jaw four premolars and only two molars while the Thylassinus has three premolars and four molars the molars also differ much in the two animals in relative size and structure the adult's dentition is preceded by a widely different milk dentition anyone may of course deny that the teeth in either case have been adapted for tearing flesh through the natural selection of successive variations but if this be admitted in the one case it is unintelligible to me that it should be denied in the other I am glad to find that so high an authority as Professor Flower has come to this same conclusion the extraordinary cases given in a former chapter of widely different fishes possessing electric organs of widely different insects possessing luminous organs and of orchids and eschlepiads having pollen masses with viscid discs come under this same head of analogical resemblances but these cases are so wonderful that they were introduced as difficulties or objections to our theory in all such cases some fundamental difference in the growth or development of the parts and generally in their matured structure can be detected the end gained is the same but the means though appearing superficially to be the same are essentially different the principle formally alluded to under the term of analogical variation has probably in these cases often come into play that is the members of the same class although only distantly allied have inherited so much in common in their constitution that they are apt to vary under similar exciting causes in a similar manner and this would obviously aid in the acquirement through natural selection of parts or organs strikingly like each other independently of their direct inheritance from a common progenitor as species belonging to distinct classes have often been adapted by successive slight modifications to live under nearly similar circumstances to inhabit for instance the three elements of land, air and water we can perhaps understand how it is that a numerical parallelism has sometimes been observed between the subgroups of distinct classes a naturalist struck with a parallelism of this nature by arbitrarily raising or sinking the value of the groups in several classes and all their experience shows that their valuation is as yet arbitrary could easily extend the parallelism over a wide range and thus the septinary, quinary, quaternary and ternary classifications have probably arisen there is another and curious class of cases in which close external resemblance does not depend on adaptation to similar habits of life but has been gained for the sake of protection I allude to the wonderful manner in which certain butterflies imitate as first described by Mr. Bates other and quite distinct species this excellent observer has shown that in some districts of South America where for instance an ethomia abounds in gaudy swarms another butterfly, namely a leptalis is often found mingled in the same flock and the latter so closely resembles the ethomia in every shade and stripe of colour and even in the shape of its wings that Mr. Bates with his eyes sharpened by collecting during eleven years was though always on his guard continually deceived when the mockers and the mocked are caught and compared they are found to be very different in essential structure and to belong not only to distinct genera but often to distinct families had this mimicry occurred in only one or two instances it might have been passed over as a strange coincidence but if we proceed from a district where one leptalis imitates an ethomia another mocking and mocked species belonging to the same two genera equally close in their resemblance may be found altogether no less than ten genera are enumerated which includes species that imitate other butterflies the mockers and mocked always inhabit the same region we never find an imitated living remote from the form which it imitates the mockers are almost invariably rare insects the mocked in almost every case abounds in swarms in the same district in which a species of leptalis closely imitates an ethomia there are sometimes other lepidoptera mimicking the same ethomia so that in the same place species of three genera of butterflies and even a moth are found all closely resembling a butterfly belonging to a fourth genus it deserves a special notice that many of the mimicking forms of the leptalis as well as of the mimicked forms can be shown by a graduated series to be merely varieties of the same species while others are undoubtedly distinct species but why it may be asked are certain forms treated as the mimicked and others as the mimicers Mr. Bates satisfactorily answers this question by showing that the form which is imitated keeps the usual dress of the group to which it belongs while counterfeiters have changed their dress and do not resemble their nearest allies we are next led to inquire what reason can be assigned for certain butterflies and moths so often assuming the dress of another and quite distinct form why to the perplexity of naturalists has nature condescended to the tricks of the stage Mr. Bates has no doubt hit on the true explanation the mocked forms which always abound in numbers must habitually escape destruction to a large extent otherwise they could not exist in such swarms and a large amount of evidence has now been collected showing that they are distasteful to birds and other insect devouring animals the mocking forms on the other hand that inhabit the same district are comparatively rare and belong to rare groups hence they must suffer habitually from some danger for otherwise from the number of eggs laid by all butterflies they would in three or four generations swarm over the whole country now if a member of one of these persecuted and rare groups were to assume a dress so like that of a well protected species that it continually deceived the practised eyes of an entomologist it would often deceive predacious birds and insects and thus often escape destruction Mr. Bates may also be said to have actually witnessed the process by which the mimicers have come so closely to resemble the mimicked for he found that some of the forms of leptalus which mimic so many other butterflies varied in an extreme degree in one district several varieties occurred and of these one alone resembled to a certain extent the common ethomia of the same district in another district there were two or three varieties one of which was much commoner than the others and this closely mocked another form of ethomia from facts of this nature Mr. Bates concludes that the leptalus first varies and when a variety happens to resemble in some degree any common butterfly inhabiting the same district this variety from its resemblance to a flourishing and little persecuted kind has a better chance of escaping destruction from predacious birds and insects and is consequently often preserved the less perfect degree of resemblance being generation after generation eliminated and only the others left to propagate their kind so that here we have an excellent illustration of natural selection Mr. Wallace and Trimman have likewise described several equally striking cases of imitation in the leperdoctor of the melee archipelago and Africa and with some other insects Mr. Wallace has also detected one such case with birds but we have none with the larger quadrupeds the much greater frequency of imitation with insects than with other animals is probably the consequence of their small size insects cannot defend themselves excepting indeed the kinds furnished with a sting and I have never heard of an instance of such kinds mocking other insects though they are mocked insects cannot easily escape by flight from the larger animals which prey on them therefore speaking metaphorically they are reduced like most weak creatures to trickery and to dissimulation it should be observed that the process of imitation is recommended between forms widely dissimilar in colour but starting with species already somewhat like each other the closest resemblance if beneficial could readily be gained by the above means and if the imitated form was subsequently and gradually modified through any agency the imitating form would be led along the same track and thus be altered to almost any extent so that it might ultimately assume an appearance wholly unlike that of the other members of the family to which it belonged there is however some difficulty on this head for it is necessary to suppose in some cases that ancient members belonging to several distinct groups before they had diverged to their present extent accidentally resembled a member of another and protected group in a sufficient degree to afford some slight protection this having given the basis for the subsequent acquisition of the most perfect resemblance