MODES OF MAMMALIAN EVOLUTION 651 



quired in a great many cases. The resultant similarity may 

 be attained through the loss, the acquisition or the modifica- 

 tion of parts. The reduction of toes from the primitive number 

 of five to four, three, two, or even one, has happened over and 

 over again in the most diverse groups. There is good reason 

 to believe that all the early and primitive placental mammals 

 had the third trochanter on the femur and the epicondylar 

 foramen on the humerus, but in most of the modern groups 

 these structures are lost ; and the list of such similar reductions 

 of parts might be almost indefinitely extended. 



Of much greater significance is the independent similar 

 modification of parts and acquisition of new structures. In- 

 numerable examples of this kind of parallel and convergent 

 development might be given, but a few will be sufficient to 

 illustrate the principle. (1) The odontoid process of the axis 

 (second vertebra of the neck) was primitively a bluntly conical 

 peg, a form which is still retained in the great majority of 

 mammals, but in the true ruminants, the camels, the horses 

 and the tapirs, the process is spout-shaped, concave on the 

 upper side, convex on the lower. By tracing the development 

 of those groups, it has been conclusively demonstrated that 

 the change of form took place independently in each of the 

 four. (2) The ruminants have molar teeth composed of four 

 crescentic cusps arranged in two transverse pairs, the pattern 

 called selenodont. The evidence is very strong that this highly 

 characteristic molar pattern has been several times inde- 

 pendently repeated, as in the true ruminants, the camels, the 

 foreodonts and probably other groups also. (3) The family 

 fMacrauchenidse of the extinct jLitopterna shares with the 

 camel tribe the remarkable peculiarity of having the canal for 

 the vertebral artery running through the neural arches of the 

 neck-vertebrae. (4) A very striking instance is afforded by 

 the three widely separated groups of hoofed animals, members 

 of which had their hoofs transformed into claws ; the fchali- 

 cotheres arose from the normal perissodactyls (p. 356), the 



