384 ORGANIC GROWTH sec. 



crossing. Its influence is highly important, but it produces 

 its results principally by the aid of crossing : by sexual 

 mixture various directions of growth already determined may 

 be united and strengthened or modified in the most various 

 ways. Whatever in the result of mixture is useful, has a 

 greater claim to exist and is capable of giving rise to new, 

 altered directions of growth. Thus the power of selection lies 

 chiefly in the promotion and diversification of organic growth. 

 It is like crossing, only an indirect cause of the evolution of 

 living beings. As was previously pointed out, sexual 

 differentiation, the necessary antecedent of crossing, has itself 

 only been developed in course of time — is itself the result of 

 peculiarities of growth in two different directions, and these 

 peculiarities of gi^owth are again primarily conditioned by 

 peculiar external influences (nourishment), and preserved by 

 selection. 



The Law of Organic Form : Its Application to the 

 Form and Structure of Plants 



In accordance with the preceding, we may formulate a law 

 of the form of living beings in the following terms : The 

 external form of every individual, every variety, species, 

 genus, family, etc., is the resultant of a number of processes 

 of growth which have taken place in its ancestors, together 

 with the effect of external conditions which have acted upon 

 it during its individual development and life, and of spon- 

 taneous internal modifications. 



In other words : The external form of every organism is 

 the result of the action of external influences on all its 

 ancestors, together with the effect of such influences and of 

 spontaneous internal modifications during its individual 

 life. 



The influence of selection is included in this definition. 



