186 THE EVOLUTION THEORY 



along with them — to two main propositions, which express the re- 

 lation of phylogeny to ontogeny. Tlie lirst and fundamental pro- 

 position is the one already formulated. The ontogeny arises from 

 J the phylogeny by a condensation of its stages, which may be varied, 

 shortened, thrown out, or compressed by the interpolation of new 

 stages. The second proposition refers to individual parts, and may 

 run as follows : As each stage can undergo new adaptations by itself, 

 so can every part, every organ ; such new adaptations very often 

 show a tendency to be transferred to the immediately antecedent 

 stage in ontogeny. 



It is not my intention to formulate the laws of ontogeny just 

 now, otherwise many others might be added to these, such as that of 

 the regular transference of characters acquired at one end of a seg- 

 mented animal to the other segments : I must confine myself here 

 to bringing the two main propositions into harmony with the prin- 

 ciples of our theory of heredity. 



How phylogeny is condensed in ontogeny can be understood 

 readily enough in a general way, although we cannot profess to have 

 any insight into the detailed processes. The continuity of the germ- 

 plasm brings about inheritance, in that it is continually handing 

 over to the germ-plasm of the next generation the determinant- 

 complex of the preceding one. Every new adaptation at any stage 

 whatever depends on the variation of particular determinants within 

 the germ-plasm, and this in its turn depends on germinal selection, 

 that is, on the struggle of the different determinant-variants among 

 themselves, and on the variation in a definite direction which arises 

 from this, as we have already shown. A new kind of determinant 

 can never arise of itself, but always only from already existing 

 determinants, and through variation of these. But as spontaneous 

 variation never causes all tlie homologous determinants of a germ- 

 plasm to vary in quite the same way, but only a majority of them, 

 there always remains a minority of the old determinants, which may, 

 under certain circumstances, predominate again, as is proved by the 

 aberi'ations in VuMesna species due to cold, and by many other kinds 

 of reversion. 



But it is not this variation which leads to tlie prolongation of 

 ontogeny, and the repetition of the phyletic stages within it. In 

 this case it is rather that a new character takes tlie place of an old 

 one, not that it is added to it. A black spot may arise instead of 

 a red one, but not first a black spot and then a red one. Of course 

 we still know far too little in reofard to the intimate succession of 

 events in the stages of ontogeny to be able to say definitely that, in 



