142 



KNOWLEDGE. 



[August 1, 1898. 



abortion of the limbs, by certain lizards, such as the 

 familiar English " blind-worm," it being very doubtful, to 

 our thinliing, if this can be explained by mimicry. Still 

 more remartcable is the external resemblance presented by 

 whales and dolphins to fishes, and by the extinct tish- 

 lizards (Ichthyosaurs) to both ; such resemblances being 

 clearly due to parallel development induced by the needs 

 of adaptation to a purely aquatic life. If, moreover, it 

 should prove that the whalebone whales have no connection 

 with the dolphins, we should then have a far more remark- 

 able instance of this feature, and one extending to internal 

 structures as well as to external form. A similar remark 

 will also apply to the case of the true seals and the eared 

 seals, which it has been suggested may be of independent 

 origin. The independent acquisition of wings by birds, 

 bats, and the extinct flying dragons, or pterodactyles, may 

 likewise certainly come under the heading of parallel 

 development, so far as external form and the adaptation to 

 a particular mode of life are concerned : although here, as 

 in the instance of whales and fishes, the structural features, 

 by the aid of which the adaptation has been brought about, 

 are very different in the respective groups. 



In respect to the teeth and dentition, many very well- 

 marked instances of parallelism may be adduced. We have 

 already referred to the similarity of the tusks in different 

 groups of mammals ; while in our last article we noticed the 

 loss of upper front teeth, not only in the modern ruminants, 

 but likewise in the peculiar even-toed imgulate known as 

 J^rotuierasf, of which the skull is figured on page 143. The 

 rhinoceroses likewise show a gradual tendency to the loss 

 of front teeth, resulting in the African forms in the dis- 

 appearance of the whole series from both jaws. More 

 remarkable, however, is the tendency to a complete loss of 

 the whole of the teeth in certain groups of all the higher 

 classes of vertebrates, as exemplified by all modern birds, 

 by turtles and tortoises, as well as certain of the extinct 

 flying dragons and fish lizards among reptiles, and by the 

 great ant-eater, the scaly ant-eater, and the echidnas, or 

 spiny ant-eaters, among mammals ; in all of which teeth 

 are completely lacking. Moreover, in many of these 

 animals, such as birds, tortoises, and probably the tooth- 

 less flying dragons, the beak-like jaws thus produced were 

 sheathed in horn, thus showing a kind of double paral- 

 lelism, viz., the loss of teeth coupled with the acquisition 

 of a horny sheath to the jaws. 



Another instance of parallel development aftbrded by 

 teeth relates to the gradual heightening of the crown and 

 the production of a flat plane of wear not only among 

 several distinct groups of hoofed mammals, such as the 

 elephants, horses, and ruminants, but likewise among the 

 rodents. In a previous article, on " Teeth and their Varia- 

 tions," it was pointed out how these high-crowned teeth 

 had been independently evolved in the three great groups of 

 hoofed mammals, of which the above-named creatures are 

 typical representatives ; and it may be added here that the 

 molars of horses and ruminants present a further evidence 

 of paiallelism in their assumption of a more or less 

 decidedly crescent-like (selenodont) structure, the essential 

 peculiarities of which are described in the article men- 

 tioned. This resemblance between the molars of horses 

 and ruminants is, however, comparatively remote, hut in 

 the latter group and the camels these teeth are so alike as 

 to require nn expert to distinguish between them. Never- 

 theless, there is good evidence to show that the camels and 

 the ruminants, if not also the chevrotains, have acquired 

 their crescent-like molar teeth quite independently of one 

 another, and it therefore yet remains for tho^e writers 

 who explain evolution by some mode of what they are 

 pleased to call natural selection to account adequately for 



the similarity thus existing between structures of such 

 totally different origin, when they could have been made 

 equally efficient if unlike. 



Passing from the consideration of teeth to that of limbs, 

 we may mention that in previous articles we have already 

 called attention to the remarkable similarity displayed in 

 the mode by which the lower segments of the limbs of the 

 even-toed and odd-toed hoofed mammals have been gradu- 

 ally elongated by the formation of a cannon-bone and 

 the disappearance of either three or four of the lateral 

 digits ; the cannon-bone in the horse consisting of but a 

 single element carrying one digit, while in the ruminants 

 it comprises two united elements supporting a pair of toes 

 — this being clearly a case of parallelism in development 

 attained by a slightly different modification of principle. 

 The parallelism does not, however, stop here, since (as 

 shown in our article entitled " Giant Birds ") an essentially 

 similar type of cannon-bone has been produced in birds ; 

 only that in that group (with the exception of the ostrich) 

 three long bones enter into its composition, which is 

 further complicated by the addition of a bone from the 

 ankle above. Seeing that in all these groups the parallel- 

 ism has been arrived at by a diti'erent structural modifica- 

 tion, the explanation of its mode of evolution is much less 

 difficult than in the case of the molars of the camels and 

 ruminants, where, as we have seen, the structure is 

 practically identical. 



Eecent discoveries in North America have brought to 

 light the existence of a kind of secondary parallelism 

 among certain peculiar mammals which may be included 

 among the hoofed or ungulate division of that class. In 



Fn)Ut and side views of tlie hind foot of Ar/ioin/r. 

 (After Oslioni.) 



the even-toed group of that division, as exemplified by 

 the pigs and ruminants, it is the third and fourth digits 

 which are symmetrical to one another, and tend to 

 develop at the expense of the others ; while in the odd- 

 toed group it is the third or middle digit which is 

 symmetrical in itself and tends to an ultra-development. 

 Now there are certain extinct creatures which, while 

 having claws instead of hoofs at the ends of their toes, 

 yet are so closely related to the hoofed mammals that 

 their separation therefrom is almost impossible. In one 

 of these, which has long been known in Europe as the 

 chalicothere, the third digit, as in the odd-toed hoofed 

 mammals, is symmetrical in itself, and larger than 

 either of the others ; whereas in the newly-described 

 animal known as Arliuni/r the third and fourth digits 

 resemble those of the even-toed division of the hoofed 

 mammals in being larger than the others and symmetrical 

 to a line drawn between them. While, therefore, we 

 have clearly a parallel development on different hnes 

 between the odd and even-toed hoofed animals in the 

 development of a cannon-bone, these extinct clawed, hoof- 



