262 



NATURE 



[January io, ipbi 



during its development, is so liable to adaptive change of but 

 passing significance, that it becomes difficult to distinguish 

 Ijetween this and the historic record originally believed to be 

 passed through. But, that notwithstanding, upon data of this 

 order many of our recent conceptions of the origin and succession 

 of animal forms have been built up ; and it is clear that if, on 

 the basis of such facts, we attempt to deduce those generalised 

 statements we term " laws," the test of our accuracy lies in 

 appeal to the fossiliferous strata, in which we ought to find evi- 

 dence of their presumed operation in the past. Here I am led to 

 emphasise the importance of the study of palseontology, and, as 

 bearing on the argument deduced from that of development, a 

 striking outcome of recent palaeontological investigation has 

 been the unearthing, in the United States of America, of per- 

 fectly preserved remains of the Trilobites, the oldest and most 

 primitive of all Crustacean forms. These creatures are now 

 proved to have been possessed of but one pair of antennas, there 

 being two pairs present in every ordinary later member of the 

 Crustacean class. It so happens, however, that this is the 

 case for the adults only, and that the presence of but a 

 single pair is characteristic of the larval stage through 

 which all freely developed Crustacea pass, and, from what is 

 now known of the details of the appendages of these Trilobites 

 and the said Crustacean larvse, there can be no doubt that in 

 this particular class of animals the larva is realistic in its 

 characters of the remote ancestor from which, in past ages, its 

 members have been derived. 



Other memorable instances have come to hand in the study 

 ■of the palaeontological record, which have profoundly modified 

 our conceptions of the succession and primary relationships of 

 animal forms. For example, evidence is now accumulating that 

 in the case of birds the remote ancestors were of a more primi- 

 tive reptilian stock than has been until recently supposed. 

 Again, those structural features in respect to which the living 

 Batrachia simplify the reptilian type are now coming to be 

 recognised as largely due to retrogressive change, and we are 

 beginning to see that both these classes of animals in all prob- . 

 ability converge towards an assemblage of palaeozoic forms, 

 combining the characters of the two as to-day represented, and 

 that the older naturalists, in classifying the cold blooded 

 terrestrial vertebrata together, were perhaps not so far out as we 

 have been prone to think. A wonderful chapter has quite 

 lately been added to the history of the horse, by the discovery 

 in South America of an equine animal which possessed the 

 single toe and other features familiar in it. The race to which 

 it belonged has apparently become extinct only in quite recent 

 times, and when we picture to ourselves the course of events to 

 which it points, we conclude that in early Tertiary times the 

 ancestors of the horse tribe, arising in Central America, migrated 

 into the Old World, on one hand, and into South America 

 on the other ; and in each, by independent but parallel 

 differentiation, gave rise to an essentially similar definitive form. 

 Survival of this in the Old World alone has resulted in the 

 horses of to-day, those now living in America having been 

 secondarily imported by man. 



The case in some respects recalls that of the pig tribe, except 

 that, with this, migration in opposite directions has been accom- 

 panied by diversity of modification. Originating in early 

 Tertiary times in Central North America, their ancestors mi- 

 grated on one hand into the Old World, and by complication of 

 their teeth gave rise to the swine and hogs of later times, while 

 on the other hand, passing into the southern parts of America, 

 they by numerical reduction of their teeth and toes gave rise to 

 the peccaries of to-day. 



Taken in conjunction with the now well-recognised fact that 

 certain animals which in life and in all superficial features re- 

 semble each other can be proved on examination of more 

 deeply-seated characters to be genetically distinct, this con- 

 sideration raises the question of the importance of what is 

 known in nature as the phenomenon of "Convergence." We 

 now know of creatures externally almost indistinguishable from 

 slugs which have the internal anatomy of snails and of slugs 

 occurring independently in different parts of the world which 

 exhibit a repetitional similarity of relationship to the snails of 

 their respective areas ; and we have long been familiar with a 

 Crustacean — the "King Crab" — living on the opposite shores 

 of the North Pacific, which, in respect to the segmentation of 

 its body and the number and characters of its limbs more 

 especially, conforms to the Scorpionid type. Numerous other 

 instances might be cited, but these are sufficient, and the 



question for consideration is, how far such superficial resem- 

 blances, in -that they have led to the association of forms> in 

 which they occur in a common classification, are trustworthy as 

 criteria of affinity. The case for the King Crab and the Scorpion 

 is one of long standing, and there is reason to believe it is still 

 open to doubt. In all groups of Arthropods, to which both 

 creatures belong, we meet with forms in which the familiar free 

 body-rings or "segments" are for the greater part united, and 

 others in which they are free, and there can be no -doubt that 

 the degree of union of these, which takes place in definite 

 antero-posterior succession, is a sure index of "highness" and 

 "lowness" in a given series— those in which few segments 

 unite being low, those in which many unite, high. To this 

 process of fusion of body-segments our American confreres apply 

 the expressive term " cephalisation," and, when this test is 

 applied to the two groups to which the animals in question 

 belong, it is found that, in respect to it and certain correlated 

 modifications, they each stand at the summit of their respective 

 series— i.e., that there are, among the " spiders," forms which, 

 at least as regards cephalisation, simplify the Scorpionid type 

 along lines parallel with those in which the so-called Euryp- 

 terids of the past, in this and other respects, simplify the King 

 Crab type ; and when, further, it is found that among the fossil 

 Scorpions known there are indicatiorts of simplification of 

 exactly the order the facts would lead us to suspect, it follows 

 that King Crab and Scorpion of to-day each hark back to a 

 distinct and independent assemblage of forms. With this, the 

 association together of the culminating types, as in most of our 

 current classifications, becomes dangerous, if not misleading ; 

 and we are brought to the realisation of the fact that mere 

 community of adult structure does not necessarily imply com- 

 munity of origin, and that by a parallelism of modification two 

 creatures of diverse ancestry, in adaptation to the conditions of 

 life, may, independently, and by "convergence," assume a 

 similar form. 



The approximate resemblance between the crowns of the 

 teeth of the horse and ox is a familiar example, and we have 

 evidence that in this way certain types of teeth represented 

 among the living mammals, by which these are still classified, 

 have been anticipated by totally different groups in past periods 

 of time ; and if we are to trust recent research, teeth already 

 modified along lines anticipatory of the carnivorous and herb- 

 ivorous types of to-day co-existed in an assemblage of supposed 

 cretaceous mammals of South America whose affinities are as 

 yet not fully established. 



The extent of the operation of "Convergence" in nature's 

 work is but now becoming recognised, and there is proof to hand 

 that many of our time-honoured classificatory systems are 

 erroneous, by failure of its appreciation in the past. Impressed 

 by this, it behoves us to reflect to what an extent nature's plans, 

 so to speak, have, in the history of organic evolution, as in that 

 of civilisation, repeated themselves, she being apparently intent 

 on a recurrent diversity of differentiation, for some purpose 

 associated with the balance of life we do not understand, as in 

 the fact that when, in Mesozoic times, she had but the reptiles 

 upon which to operate, she produced terrestrial, aquatic and 

 flying forms, just as, in later periods, she has produced them 

 with the mammals, which replaced these in order of time. 



Again, with the development of the Darwinian doctrines, 

 there early arose the realisation that, on the principle of descent 

 with modification, the summary appearance of organs having no 

 existence in near allies, either during the development of certain 

 species or on the assumption of the adult state, presented a 

 difficulty which even Darwin himself, ever more justly critical 

 of his own work than many of his would-be opponents, clearly 

 admitted. The independent appearance of luminous organs 

 and those of electrical discharge in remotely related groups of 

 fishes are ideal cases, at first sight calculated to break the 

 back of the rigid Darwinian. Some thirteen years ago. Dr. 

 Anton Dohrn, of Naples, and the late Prof. Kleinenberg, of 

 Messina, formulated the doctrine of "Substitution of Organs," 

 which provides that under varying conditions of life, and at 

 different periods of development, sets of organs may replace 

 others, to ihe belter fulfilment of the life of the individual or 

 race. To apply this to the case of the organs of electrical dis- 

 charge in fishes in no way closely related, let it be said that our 

 commonest rays and skates are possessed of such an organ, 

 located in the tail. The peculiar feature of these fishes is the 

 usurpation by their expanded side-fins of the propelling action, 

 which, in ordinary fishes, is performed by the tail. What more 



NO. 1628, VOL. 63] 



