698 Studies in the Theory of Descent. 



of nature, and if we consider both the former and 

 the latter to work together with equal power. 

 Von Hartmann expressly makes such an admission 

 under the name of an " internal principle of de- 

 velopment," to which he attributes such an im- 

 portant share that one cannot understand why it 

 should have any need for the employment of causal 

 powers, and why it does not simply do everything 

 itself. Von Baer expresses himself much less 

 decisively, and even in many places insists upon 

 the purely mechanical connection of organic 

 natural phenomena ; but that with him also the 

 idea of interruption by a metaphysical principle is 

 present, is principally shown by his assuming, at 

 least partly, the per saltum development of species. 

 This necessarily involves an actively internal 

 power of development. 



Although I have already brought forward many 

 arguments against the existence of such a power, 

 and although in refuting it every form of develop- 

 ment by directive powers is at the same time 

 overthrown, it nevertheless appears to me not to 

 be superfluous in such a deeply important question 

 to show that a per saltum development, and 

 especially the so-called heterogeneous generation, 

 is inconceivable, not only on the ground of the 

 arguments formerly employed against thephyletic 

 vital force in general, but quite independently of 

 these. 



In the first place it must be said that the 



