On the Afcc/ianieil Conception of Nature. 683 



change would be different according to its nature, 

 or, in other words, organisms of different natures 

 react differently when affected by equal modifying 

 influences. The physical nature of the organism 

 plays the chief part with respect to the quality of 

 the variations ; each specific organism can thus 

 give rise to extremely numerous, but not to all 

 conceivable, variations ; that is, only to such vari- 

 ations as are made possible by its physical 

 composition. From this it follows further that 

 the possibilities of variation in two species are 

 more widely different, the wider they diverge in 

 physical constitution (including bodily mor- 

 phology) that a cycle of variation is peculiar to 

 every species. In this manner we are led to the 

 knowledge that there must certainly exist a " fixed 

 direction of variation," but not in the sense of 

 Askenasy and Von Hartmann, as the result of an 

 unknown internal principle of development, but as 

 the necessary, i. e. mechanical, consequence of the 

 unequal physical nature of the species, which 

 must respond even to the same inciting cause by 

 unequal variations. 



The facts, as far as we know them, agree very 

 well with this conclusion. Allied species vary in 

 a similar manner, whilst species which are more 

 distantly related vary in a different manner, even 

 when acted upon by the same external influences. 

 Thus, in the first part of these " Studies " I have 

 remarked that many butterflies under the influence 



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