794 Comparative Morphology of Chordates 



be accounted for by mere increase in size of the body. It was probably 

 in the Glacial Period (one or two hundred thousand years ago) that 

 some of these larger primates forsook the trees and came down to 

 earth. But they retained their long arms and gradually adopted bipedal 

 locomotion on the hindlegs with the body in a more or less nearly 

 upright position. With the prehensile digits they grasped clubs or 

 stones and, in the course of time, crude weapons and tools. Such, accord- 

 ing to the fully consistent evidence from anatomy, embryology, and 

 paleontology, was the origin of the human primate which, in an in- 

 credibly short (in the geologic scale) time, has become the world's 

 dominant annual. 



Lines of Specialization 



Certain aspects of this history of vertebrates require special em- 

 phasis. Throughout the history (true not only of vertebrates but of all 

 animals) may be seen numerous instances of lines of descent diverging 

 from some common ancestral type. "Divergence" expresses the fact 

 that along each of the several lines of descent the adaptive modifica- 

 tions or "specializations" of the ancestral structure are of a sort 

 peculiar to that one line. Each of several such diverging lines may be 

 called a "line of specialization." Degree of specialization is measured 

 by the extent to which an organ or animal has progressed along its 

 own peculiar line of specialization, whether the line be long or short. 

 The mammalian ear is very highly specialized compared to a shark's 

 ear. The mammary organs of a placental mammal are highly special- 

 ized compared to those of monotremes. Among Carnivora, seals have 

 gone much farther than others along the line of specialization for 

 aquatic life. Specialization, broadly defined, may consist either in 

 elaboration and complication of a structure, or in reduction of it. 

 The primitive pentadactyl mammalian foot is more highly specialized 

 in the one-toed foot of a horse than in the three-toed foot of a tapir. In 

 most snakes specialization by reduction of the paired appendages has 

 gone to its limit, complete obliteration. 



"Primitive" and "specialized" are relative terms. An animal which 

 is a primitive member of its own group may, in some particulars, be 

 specialized in relation to members of the group from which it was 

 derived. Archaeopleryx was a primitive bird, but it had made consider- 

 able progress along the avian line of specialization as contrasted to its 

 reptilian progenitors. 



An especially significant fact which emerges from the history of 

 vertebrates is that new lines of specialization arise, not from some 

 highly specialized division of an established group, but always from 

 some relatively nnspecialized members of it. Amphibians arose, cer- 



