ANIMAL EVOLUTION 7 



substances, gradually increasing as the Earth has assumed 

 its present form, increased in a transcendent degree 

 during that stage which preceded the origin of life.' 



Thus evolution was in progress long before life appeared 

 on the globe, and the origin of life is due to the con- 

 tinuance of the process of the formation and recombina- 

 tion of more and more heterogeneous molecules. To 

 explain the transition from non-living to living matter 

 he assumed the coming into existence of ' physiological 

 units '. 



' There seems no alternative,' he wrote, ' but to 

 suppose that the chemical units combine into units 

 immensely more complex than themselves, complex as 

 they are : and that in each organism the physiological 

 units produced by this further compounding of highly 

 compound molecules, have a more or less distinctive 

 character. We must conclude that in each case some 

 difference of composition in the units, or of arrangement 

 in their components, leading to some difference in their 

 mutual play of forces, produces a difference in the form 

 which the aggregate of them assumes.' 



We must take note of the fact that the physiological 

 units are assumed to differ among themselves from the 

 beginning, and that the interactions, resulting from 

 these differences of chemical composition, induce further 

 differences, which are expressed in the structure of the 

 different parts of an organism formed by an aggregate 

 of such units. Starting from this conception, it is 

 a comparatively easy task to picture a gradual increase 

 of complexity in the organic world, due on the one 

 hand to the combinations of different kinds of units 

 into aggregates, and the changes induced at once in 

 the units and in their aggregates by the action and 

 reaction of the units on one another. Due on the other 

 hand to the action of incident forces, gravity, heat, light, 

 electricity, chemical stimuli, mechanical stimuli, both 

 on the units and on their aggregates, inducing change. 



