EVOLrXION OF THE VERTEBRATA. 315 



organs of movement have much to do with the question. When 

 perfection in this respect conflicts with perfection of brain, in 

 evidence of position, we naturally give the preference to the 

 latter in deciding. Thus the ruminating mammals are much 

 superior to man in the structure of their feet, teeth, and stomach, 

 yet we properly assign the higher position to the Quadrumana and 

 to man, on account of the superior complication of their brain- 

 structure. 



Paleontology has proved,* what had been already surmised, 

 that the development of animal organisms has been on lines of 

 increasing specialization of parts. That is, in lines of increas- 

 ingly perfect adaptations of structures to ends, or functions. In 

 certain series of animals we witness steadily increasing perfection 

 of mechanisms of the limbs for running ; in others for digging ; 

 in others for flying. In the teeth we find increasing perfection of 

 machines for grinding, for cutting, or for seizing. In the brain 

 the specialization has evidently been toward increased acuteness 

 of perception, increased energy of action, and increased intelli- 

 gence. Specialization does not, however, necessarily imply pro- 

 gressive development. Adaptation may be to a parasitic or a ses- 

 sile mode of life. Such adaptation is often displayed in a very 

 special modification of parts, as in the anterior limbs of some of 

 the parasitic Crustacea ; in the mouth parts of some Arachnida ; 

 in the feet of the sloth, and in the jaws of the ant-eaters. 



Embryology has furnished, and will furnish, many important 

 hints and demonstrations as to the true meaning of the rudiment- 

 ary condition or absence of parts, and thus indicate the phylo- 

 genetic connections of animals. Thus the origin of the Tunicata 

 from primitive vertebrate-like forms would probably never have 

 been suspected but for embryological studies ; and the origin of 

 the very peculiar order of Eotifera has been explained in like 

 manner. But embryology has its limitations, for the transitional 

 characters presented by embryos are only partially of the nature 

 of a record of the structures which belonged to their ancestors in 

 successive geological ages, and are frequently special adaptations 

 to the necessities of their embryonic life. Such are the stato- 

 blasts which are present in fresh-water sponges and Polyzoa, and 

 wanting in the marine forms ; and the allantois and placenta of 



* Cfr. " On the Evidence for Evolution in the History of the Extinct Mam- 

 malia," "Proc. Amer. Assoc. Adv. Sciences" for 1883. 



