372 SCOTT. [Vol. V. 



and retrogression have occurred. Of course, a mammal may 

 advance to a certain degree and then degenerate, but that is 

 not the point under consideration. Nor is it meant that the 

 direction of advance is something absolutely fixed ; on the con- 

 trary, this direction may be followed for a certain length of time, 

 and then changed. A very instructive example of this is given 

 by the history of the Pecora. The lower miocene representa- 

 tives of this series, which are hornless, AmpJiitraguliis, Palceo- 

 meryx, etc., show a continually increasing size of the upper 

 canines, which are still retained in the hornless deer, Afoschus, 

 Hydropotes, etc., as well as in the small-antlered muntjaks, while 

 in the typical Cervidce and in the Bovidce the antler and the 

 horn have been gradually substituted as weapons, and the upper 

 canines have dwindled to mere rudiments or entirely disappeared. 

 CJialicotherium is doubtless an example of the change of direc- 

 tion of specialization. 



Obviously, this conclusion is far from certain, and may well 

 be modified or even disproved by more extensive and com- 

 plete material. Nor can we extend this result to the lower 

 groups of animals. The facts which have been brought out with 

 regard to the Axolotl, the experiments of Schmankewitsch 

 (No. 48) on Artemia, and many other facts, render it inapplicable 

 to these forms. But it appears to be the fact that, just as the 

 power of regeneration of lost parts diminishes as we ascend in 

 the scale of animal life, so plasticity of organization. and capacity 

 for differentiation of structure in widely different directions 

 diminishes also. 



6. As a general rule, it certainly appears to be the case that 

 among mammals differentiation is by reduction in the number 

 of parts, but there are some facts which go to show that this rule 

 is not without exceptions. Thus in the toothed whales and in 

 certain edentates the number of the teeth has surely been in- 

 creased, as has the number of the phalanges in the Cetacea and 

 the Sirenia. Probably no one will be inclined to believe that 

 the primitive antelopes had four horns and that Tetraceros is the 

 only living form which has retained this peculiarity. On the 

 contrary, it seems certain that in this genus an additional pair 

 of horns has been developed, just as in certain monstrous do- 

 mestic breeds of sheep. In certain breeds of dogs a double 

 hallux has been rendered constant. Indeed, reduplication of 



