40 



TITANOTHEEES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



chemical explanations; but it appears that we may 

 be led to the adoption of certain physical conceptions 

 and the use of certain physical terms (Osborn, 1917. 

 462) for what has been described above as the rate of 

 evolution of certain characters as distinguishing 

 genera. For the term "rate" we will substitute the 

 term "velocity." 



Ontogenetic velocity. — The velocity of the evolution 

 of certain characters in embryonic development — in 

 fact, throughout the whole course of individual 

 development — has long been a very familiar feature 

 of adaptation. From the embryo onward a char- 



isms, and we shall see that the most plausible explana- 

 tion of it thus far offered is the theory of natural selec- 

 tion proposed by Darwin. 



Phylogenetic velocity. — Another kind of velocity, 

 however, may be noted in the perissodactyls and may 

 be measured and calculated with great precision in 

 the numerous phyla of titanotheres here considered. 

 This velocity may be called phylogenetic velocity. Its 

 postulation rests upon the fact that a given character 

 may evolve much more rapidly in the members of one 

 phylum than in the members of a related phylum, al- 

 though the environment of both phyla may be the 



FiGiTRB 32. — The family tree of the perissodactyls, showing adaptive radiation of the nine families and thirty- 

 five subfamilies 



Exhibiting their divergence In limb and foot structure into cursorial, forest-hving, mediportal, and graviportal types and in tooth structure into 



browsing and grazing types. 



acter may be either hurried along or slowed down in 

 its rate of development, and in consequence it will 

 appear in earlier or later stages of individual life. For 

 example, certain adult proportions of the limbs are 

 needed at birth in all cursorial animals; these adult 

 proportions are consequently hurried forward during 

 the foetal life, so that the animal is at birth able to 

 run immediately with almost the same speed as the 

 parent. This kind of velocity of development is 

 called ontogenetic; it is appurtenant to every char- 

 acter in every stage of its development, it is closely 

 connected with the survival of certain young organ- 



same. For example, in twelve subfamilies of titano- 

 theres we observe homogeneous characters evolving 

 independently — the same cusps on the teeth, the same 

 horns on the skull. How, then, do the subfamilies 

 differ from one another? They differ because the 

 evolution of each character in each phylum proceeds 

 with its distinctive velocity. In a phylum that is 

 evolving rapidly a certain character appears early in 

 geologic time; in a phylum that is evolving slowly the 

 same character appears late in geologic time. The 

 titanotheres of one phylum may at a particular geologic 

 period be completely hornless, whereas those of a con- 



