26 



KNOWLEDGE & SCIENTIFIC NEWS. 



[Mar., 1904. 



Perhaps, however, tht- ni'-i important peculiarity in 

 the skeleton of the camels (and it will be unncessary on 

 this occasion to refer to the soft-parts) is to be found in 

 the vertebra- of the neck, which are unusually elongated. 

 In all other mammals, with the exception of the extinct 

 South American macrauchenia, the canal for one of the 

 great arteries for the neck perforates the process pro- 

 jecting from each side of the \ertebra' : but in the 

 camels and macrauchenia it runs obliquely through the 

 side- wall of the tube for the spinal marrow. 



Fig. 1.— Front Cannon-Bone of a Camel. 



All these features combined ser\'e to show that the 

 camel tribe is widely separated from the true ruminants. 

 How far back we have to go before we come to the 

 common ancestral stock is indeed at present uncertain. 

 Possibly both groups are independently derived from 

 primitive ungulates in which tlie cheek-teeth had not yet de- 

 veloped a crescentic type of structure. lie this as it may, it is 

 quite certain that the ancestral camels had low-crowned 

 cheek-teeth, comparable to those of the ancestral horses. 



Indeed, making due allowance for the fact that in the 

 one case the modification has been carried on the artio- 

 dactyle, and in the other on the perissodactyle plan (that 

 is to say, with the enlargement of the third and fourth 

 toes, instead of the third alone), the evolution of the 

 camels has followed much the same lines as that of the 

 horses. And this is only whatjmight have been expected, 

 since, as stated in the previous article of this series, it is 

 only on such lines that we can conceive evolution of this 

 nature to be possible. 



/\s examples of this general similarity, or parallelism, 

 I may refer, in the first place, to the enormous 

 increase in bodily size which has taken place. Equally 

 noticeable is the elongation of the bones of the lower 

 segments of the limbs, coupled with the tendency to 

 do away with double bones in such of those seg- 

 ments as they exist, the suppression of the lateral 

 digits, and the enlargement of those which remain. In 

 both cases there is likewise a progression from low- 

 crowned to tall-crowned cheek-teeth, and in both the 

 development of a bar of bone beyond the eye so as to 

 enclose its socket in a complete bony ring. 



The combination of all these factors tends (in addition 

 to the augmentation of bodily size) to increase the 

 speed and the longevity of the animals, and at the same 



time to render them fitted to subsist on the vegetation 

 characteristic of the present and immediately preceding 

 epochs ; the strengthening of the limbs so as to enable 

 them to support the increased weight, and at the same 

 time to withstand the strain of the increased speed, 

 being, of course, an essential feature of the process. 



.^part from certain still older and more primitive mam- 

 mals, with teeth of the tubercular type, the earliest known 

 form which can definitely be included in the camel series 

 is Piotylopiis, of the Llinta, or Upper Eocene period of 

 North America. In this creature, which was not larger 

 than a European hare, there was the full typical number 

 of 44 teeth, which formed a regular series, without any 

 long gaps, and with the canines but little taller than the 

 incisors, while the hinder cheek-teeth, although of the 

 crescentic type, were quite low-crowned. In both jaws 

 the anterior front teeth were of a cutting and compressed 

 type. Unfortunately, the skull is incomplete, and the 

 rest of the skeleton very imperfectly known ; but sufficient 

 of the former remains to show that the socket of the eye 

 was open behind, and of the latter to indicate that in the 

 hind foot, at any rate, the upper bones of the two func- 

 tional toes had not coalesced into a cannon-bone. The 

 lateral hind toes (that is to say, the 2nd and 5th of the 

 typical series) had, however, already become rudimentary ; 

 although it is thought probable that the corresponding 

 digits of the fore-limb were functional, so that this foot 

 was four-toed. \'ery remarkable is the fact that in old 

 individuals the bones of the fore-arm (radius and ulna) 

 became welded together about half-way down, although 

 they remained free above. (_)n the other hand, it appears 

 that the smaller bone of the leg (fibula) was welded to 

 the larger one (tibia), and that itp upper portion had dis- 

 appeared. Nothing is known of the neck-vertebra'. It 

 is, of course, evident that there must have been an earlier 

 form in which all the feet were four-toed, and the bones 

 of the fore-arm and lower part of the leg separate. 



A stage higher in the series, namely, in the Oligocene, 

 we meet with the much" better known Porhiotlieriuni, the 

 skull of which (fig. 2) was described so long ago as i>^\'/. 

 In this animal, which is also American, a distinct increase 



Fig. 2.— 5lvuli of Poebrotlierium 



in bodily size is noticeable, as is also one in the relative 

 length of the two bones which unite in the higher types 

 ot form the cannon-bone. Moreover, the crowns of the 

 hinder cheek-teeth are rather taller and more distinctly 

 crescentic, both feet are two-toed, the ulna and radius 

 were fused, and the fibula was represented only by its 

 lower part. In the vertebra^ of the neck the distinctive 



