204 CAUSES OF VARIATION 



systems from nebulae. The ultimate action of vital units in each other's pres- 

 ence would be no more, nor less, of a mystery than the ultimate action of 

 material units. . . . The real objection to bathmic evolution lies not in any 

 a 'priori reason against an inherent growth force, but to the obvious histor- 

 ical fact that such a force has been used to cover all sorts of obscure reason- 

 ing and even sheer foolishness. Science would welcome above all things a 

 description of the action between vital units as simple as the law of gravi- 

 tation, provided it gave a causal account of variation; and the welcome 

 would be none the less sincere if the action showed that variation was biased 

 and that evolution would be irreversible even with a reversed sequence of 

 physical environments. 1 



Cope's theory of acceleration or retardation of growth force 

 is of course merely quantitative, and would explain any differences 

 that might arise through either size or proportions of parts, or 

 faculties dependent upon such proportions. It does not attempt 

 to explain the introduction of characters, and if it can be in any 

 way controlled no method has yet been pointed out. 



We all allude to the same general thought when we use the 

 words " vitality " and " constitution " to denote not so much ten- 

 acity of life as vigor of growth, and we all recognize that some 

 individuals and some lines possess this faculty in much higher 

 degree than others. Some individuals never survive the embry- 

 onic stage ; others die in infancy ; still others reach full maturity, 

 and a few persist to an advanced age. As death comes only with 

 the failure of some vital function, the individual may persist long 

 after he is stripped of everything that makes life enjoyable. 



It is so with races. Some seem endowed with phenomenal 

 vigor, while others are preserved from extinction only with the 

 greatest difficulty and by the narrowest margin, not only because 

 of low fertility but also by reason of inherent lack of vigor. The 

 existence of these internal forces is not a matter of doubt, and their 

 office in directing variation is an interesting and valuable problem 

 which the present state of knowledge is insufficient to solve. 



Orthogenesis.* Closely akin to Cope's conception is Eimer's 

 theory of orthogenesis 2 (straight creation), or, as. he calls it, 

 "definitely directed evolution." 



1 Pearson, Grammar of Science, pp. 375-376. 



3 Eimer, On Orthogenesis and the Importance of Natural Selection in Species- 

 Formation (pamphlets, 56 pages) [Open Court Publishing Company]. 



