152 HEREDITY. 



There are five theories for the explanation of the 

 origin of the diversity of forms in animals and plants 

 and all that has life. Turning from the metaphysical 

 side of the question as to the origin of necessary- 

 beliefs, I now am to outline before you the principal 

 theories on the physiological side of that problem in 

 philosophy. 



Hereditary descent has been explained by one or 

 the other of these hypotheses : — 



1. Chemical affinities ; 



2. Elective affinities ; 



3. Organic polarities ; 



4. Inherent movements in bioplasm ; 



5. Life, defined as the power which co-ordinates 

 the movements of germinal matter. 



We have, in the first place, the old Lucretiau 

 hypothesis, or atomic theory, that chemical affinities 

 and physical forces explain the origin of form in 

 organisms. In the name of Herbert Spencer himself, 

 we may make short work with that style of material- 

 ism. Agassiz used to say that if only physical and 

 chemical forces are at work in the organisms of 

 plants and animals, we cannot account for the diver- 

 sity of the types of growth. The chemical units 

 are the same throughout the world. Ox} r gen is 

 oxygen in the elm and in the palm, in the eagle and 

 in the lion. Hydrogen, carbon, as ultimate atoms 

 are the same throughout the world, and, for all we 

 know, throughout the universe ; and therefore there 

 is no accounting for the diversity of form in organi- 

 zations if physical forces are the only ones at work 



