26 THE ORGANIC GROWTH OF THE LIVING WORLD sec. 



ment the stages of growth of the organic world. The 

 repetition in the development of the individual, in ontogeny, 

 of the ancestral development, of phylogeny, consists at the 

 same time in the compressed abbreviated exhibition of the 

 characters acquired by the whole series of ancestors and 

 transmitted to the developing individual. Phylogeny is the 

 mechanical cause of ontogeny. 



If the organic world is a connected whole, the same 

 fundamental laws must hold for all the members as for the 

 whole — therefore also the law of growth. 



If all members of the organic world are either directly or 

 indirectly connected by affinity, have been derived one from 

 another, — if, according to the biogenetic law, the development 

 of each single being consists in its growing on through stages 

 which represent the series of its forefathers (stages of growth 

 from the cell to the vertebrate, for example), — then, on the 

 assumption that this individual development morphologically 

 and physiologically forms a repetition of the ancestral evolu- 

 tion, the proof is afforded by the biogenetic law alone that 

 even the highest organisms, that the organic world in general, 

 in the course of ages, has grown up from cells. 



If the organic world is thus a connected whole, as bio- 

 logical research now assumes, and if it has grown up as I am 

 here attempting to prove, then two further important questions 

 have to be faced, namely: (1) What causes have brought 

 about a separation of this organic world (whose forms of 

 their own accord would be in uninterrupted connection, 

 would be united by imperceptible transitions) into different 

 members, into kinships — into species, genera, etc.? (2) To 

 what causes is it due that any given highest species in a 

 group of related species — assuming divergent branching in 

 the tree of descent — is a stage farther evolved than those 

 next to it ? 



I will first endeavour to answer the first of these ques- 



