ANIMAL EVOLUTION 13 



is much the most important of the two, and the nature 

 of the conditions. The direct action of changed conditions 

 leads to definite or indefinite results. In the latter case 

 the organization seems to become plastic and we have 

 much fluctuating variability. In the former case the 

 nature of the organism is such that it yields readily, when 

 subjected to certain conditions, and all or nearly all the 

 individuals become modified in the same way.' ^ 



Darwin had no doubt that variations, produced in 

 this way, are inheritable, but he did not attach great 

 importance to the direct action of external conditions 

 as a means of producing modifications of structure, 

 except in so far as they cause variations to arise and thus 

 afford the material for the action of Natural Selection. 

 I have quoted this passage because it contains reference 

 to the distinction between ' the nature of the organism, 

 which is much the most important of the two, and the nature 

 of the conditions '. 



Darwin, in a word, attached much more importance to 

 the character of the organism throughout its whole 

 history from the germ to the adult condition : Spencer 

 attached much more importance to the direct modifying 

 action of external conditions. As Weismann has pointed 

 out, while Spencer gave Sin(epigenetic, Darwin gave an 

 evolutionary account of development, and invented the 

 theory of pangenesis to account for the great complica- 

 tion of germinal structure necessary for the explanation 

 of an evolutionary process in ontogeny. 



The term ' evolutionary ' as applied to ontogeny bears 

 almost exactly the opposite meaning to that which it 

 bears when applied to phylogeny. In the latter case it 

 means a progress from the simple and undifferentiated to 

 the complex and differentiated : in the former case it 

 means the gradual unfolding and manifestation of a pre- 

 existing complexity. 



Darwin saw clearly that such a complexity must exist 

 ' Origin of Species, 6th ed., 1880, p. 106. 



