APPENDIX 431 



But there is another point to notice. We find that the 

 evolution of marking proceeds also in a definite direction in 

 this respect ; that as a rule modification extends from behind 

 forwards, so that when the male assumes a new stage of 

 marking it appears first at the posterior end of the body. 

 Thus we may see in birds of prey that the tail first shows 

 the transverse striping, while in the middle of the body we 

 have spots, and on the anterior part, the head and neck, there 

 are still longitudinal stripes. When we place a whole series 

 of allied species alongside one another we can see which is 

 the most advanced in evolution, and we may possibly detect 

 in this species the traces of a new marking on the most 

 posterior part of the body of the male. In the course of 

 generations the new marking advances forwards, then still a 

 new one begins behind and passes forwards, and at last a 

 trace of the first is only to be found remaining on the most 

 anterior part of the body of any descendant, and only in a female 

 (postero-anterior evolution). A similar progress of modification 

 clearly takes place in many animals in the direction from the 

 lower to the upper side of the body (infero-superior evolution). 



In this way we can recognise that a definite evolution 

 passes over the individual forms in a definite succession of 

 stages by a natural necessity. 



This conclusion is in a certain sense opposed to Darwin's, 

 since it recognises a perfectly definite direction in the evolu- 

 tion and continuous modification of organisms, which even 

 down to the smallest detail is prescribed by the material 

 composition (constitution) of the body. According to this 

 conclusion, the real Darwinian principle, that of selection 

 depending on utility, is only effective within the limits which 

 are prescribed by the material composition of the body, that 

 is, by the fixed directions of evolution. 



Accordingly there is nothing fortuitous, but everything in 

 evolution to the smallest detail is governed by laws. 



