NOVEMBEB 29, 1907] 



SCIENCE 



747 



eral evidence and further examination by 

 others. 



First: That such origins are adaptive in 

 direction from the ieginning. The cusps 

 of the teeth of mammals offer a peculiarly 

 advantageous field of observation because 

 they are born complete, and, unlike most 

 other organs of the body, they do not de- 

 pend upon ontogeny for their perfection, 

 in fact, ontogeny and environment destroy 

 rather than perfect them. In seventeen 

 orders of mammals, in thousands of species, 

 and in millions of individuals, a very lim- 

 ited number of similar cusps rise in the 

 teeth; the number is eleven in all. So far 

 as observed: (1) they rise independently, 

 (2) they rise gradually, (3) they rise 

 adaptively; hence I have termed them 

 " rectigradations, " i. e., rising continuous- 

 ly, orthogenetieally, in definite or straight 

 lines, and finally reaching a condition in 

 which they may be considered adaptive. 

 This phenomenon I first observed in the 

 teeth and later in the origin of horns. 



Second: That such origins are prede- 

 termined hy hereditary kinship. This 

 statement, or rather hypothesis, is sup- 

 ported by observations of two kinds. 

 Without interbreeding, animals of similar 

 kinship, near or remote, in different parts 

 of the world originate independently sim- 

 ilar characters. For example, the Eocene 

 Equidffi evolved the same cusps in the 

 grinding teeth simultaneously in Switzer- 

 land and in the American Rocky Moun- 

 tain region. 



This example has to meet the criticisms, 

 (1) by the paleontologist Deperet, that this 

 is not an independent evolution,' but that 

 these rectigradations are due to actual com- 

 munity of descent brought about by migra- 

 tion and interbreeding; (2) there is the 

 older criticism of the selectionists, that 

 these similarities are due to the similar 

 action of natural selection working upon 

 fortuitous variations in different regions. 



Neither of these explanations is tenable, 

 in my opinion. 



Third : This predetermination is due to a 

 similarity of hereditary potential. That is 

 to say that animals of similar kinship do 

 not continuously evolve in certain direc- 

 tions, but merely transmit a similar poten- 

 tiality in the origin of new characters. 

 This both renders possible the occurrence 

 of certain characters and conditions or 

 limits these characters when they do occur. 

 For example, in a certain series of extinct 

 mammals we can predict where a new cusp 

 will arise before its actual occurrence. 



As to these three propositions, which are 

 enormously important, if true, we make six 

 notes. 



We note (1) that only through some re- 

 straining or limiting law of this kind can 

 we explain the marvelous uniformity in 

 the fundamental structure of the teeth of 

 mammals which has now been observed in 

 all orders of mammals except four. 



We note (2) that this is not identical 

 with the internal perfecting tendency of 

 Nageli, because under the law of the four 

 inseparable factors, it operates, in a manner 

 adaptive to new conditions which is en- 

 tirely incomprehensive to us. Thus, for 

 example, if a primate (a monkey or lemur) 

 begins to imitate the habits of an ungulate 

 by becoming herbivorous, it also begins to 

 acquire the dental cusps of an ungulate in 

 about the same order as these cusps would 

 arise in an ungulate; thus some of the 

 Eocene monkeys so closely paralleled the 

 Eocene ungulates in dental structure that 

 they were at first placed in the order Un- 

 gulata. 



We note (3) that the kinetogenesis or 

 Neo-Lamarckian theory of Cope and Ryder 

 apparently fails (as pointed out by Poul- 

 ton), especially when applied to the teeth, 

 because the teeth appear through the gums 

 fully formed and are not modified or im- 



