338 ORGANIC GROWTH sec. 



have been produced. Further, " the more definite, the more 

 powerful and continuous, and the less modified by other 

 influences the actions of the external world on given 

 organised materials, the more similar will be the structural 

 forms which they produce, even when the blood-relationship 

 of these materials is by no means close. 



" Such an action is present in those definite unchangeable 

 physical influences which determine the origin of the sense- 

 organs in a given formative material, in consequence of the 

 necessary requirements of adaptation. The power of adapta- 

 tion, in comparison wdth that of heredity, here comes into 

 extraordinary prominence. The influences of the former are 

 very powerful. Even where the manifold slighter relations of 

 life between individuals come into play reciprocally, these 

 relations after repeated action have a great effect in changing 

 forms, and when long continued are important agents in modi- 

 fication. But the influences which primarily govern every 

 organism in consequence of the necessities of its existence, 

 the physical influences of the media in which it lives, con- 

 stitute forces which, though simple, are counteracted by no 

 opposing factors, which are ever powerfully acting in the same 

 way, and to which, from the given material, only a limited 

 variety of structural modifications can be adapted. And there- 

 fore, in spite of the endless variety in detail, in particulars, a 

 certain uniformity of organisation in the gross must exist : 

 thus the same plans of structure may appear repeatedly 

 where there is no close blood -relationship to account for 

 them. Since the continuous influence of elementary physical 

 forces necessarily exhibits itself most in the organisation of 

 the sense organs, a comparative anatomy of these organs, 

 founded essentially upon heredity, can only be established 

 usually within the smaller divisions of the animal kingdom, 

 and among all others their similarities of form are the least 

 trustworthy guides to phylogenetic relations." 



