The Theory of Evolution 81 



so in a certain degree are recapitulated. We also admit that 

 the embryonic forms of higher animals have many points of 

 comparison with the mature forms of related groups standing 

 lower in the system. 



" Nevertheless, a deeper insight into the conditions re- 

 lating to these resemblances shows that there are very im- 

 portant differences that should not be overlooked. Three 

 points need to be mentioned : i . The cell-material which in 

 the ancestral chain gives the basis for each ontogenetic process 

 is each time a different material as far as concerns its finer 

 organization and primordia. Indeed, the differences become 

 greater the farther apart the links of the original chain 

 become. This thought may be formulated in another way : 

 The same ontogenetic stages that repeat themselves periodi- 

 cally in the course of the phylogeny always contain at bottom 

 a somewhat different cell-material. From this the second rule 

 follows as a consequence. 2. Between the mature end-form 

 of an ancestor and the corresponding embryonic form of a 

 widely remote descendant (let us say between the phylo- 

 genetic gastraea and the embryonic gastrula stage of a living 

 mammal, according to the terminology of Haeckel) there 

 exists an important difference, namely, that the latter is sup- 

 plied with numerous primordia which are absent in the other, 

 and which force it to proceed to the realization of its develop- 

 mental process. The gastrula, therefore, as the bearer of 

 important latent forces, is an entirely different thing from the 

 gastraea, which has already reached the goal of its development. 

 3. In the third place, at each stage of the ontogeny outer and 

 inner factors are at work, in fact even more intensely than 

 in the fully formed organism. Each smallest change that acts 

 anew in this way at the beginning of the ontogeny can start 

 an impulse leading to more extensive changes in later stages. 

 Thus the presence of yolk and its method of distribution in 

 the egg alone suffice to bring about important changes in 

 the cleavage, and in the formation of the germ-layers, the 



