382 ORGANIC GROWTH sec. 



stage of pliyletic growth, since the latter, wherever we con- 

 template it, represents a sum of individual growths, both 

 are traced back to one and the same process — fundamentally 

 they cannot be separated. 



Phyletic growth, or the evolution of the organic world ever 

 into higher and more complex forms, or at least into forms of 

 different structure, is, as I have said, merely the sum of the 

 processes of growth of the ancestors — together with the 

 result of external influences on the forms during their develop- 

 ment and their existence. This additional modification which 

 the individuals as such undergo is — together with the influence 

 of crossing — the very cause of the constantly progressing 

 evolution. All that the members of a series of individuals 

 directly connected by descent acquire constitutes together 

 the material for the formation of new species. 



Individual growth was described above as a process which 

 according to constant laws forms permanent conditions, or con- 

 ditions which when they are transient constitute stages in the 

 further development in the same direction. The same was 

 previously asserted of phyletic gi^owth ; it forms permanent 

 stages which serve as the basis for further development, and 

 which thus render progression (or retrogression) possible. 



The variety of the external conditions, i.e. of the external 

 stimuli (including food) and of the constitution, together with 

 crossing, necessarily led to variety of growth, to variety of 

 structure, necessarily led to the formation of a variety of 

 species, provided that separation of the continuous chain of 

 forms into separate links took place. 



Thus we can appropriately speak of the organic growth of 

 species. 



The reader should compare the previous propositions with 

 the arguments of Section 11.^ I have there made special 

 use of the biogenetic law as evidence that the w^orld of 



1 Pp. 21, 25, 51, etc. 



