OTHER FACTORS OF EVOLUTION 137 



tant factors for ever co-operating with Organic Polarity and Natural 

 Selection as causes of Evolution, and as producers and maintainers 

 of new organic forms. And if such causes have ever been in opera- 

 tion the rate of Evolution must have been very notably greater 

 than that which would have resulted from the sole action of a 

 process like Natural Selection, which, from its very nature, is one 

 that must of necessity bring about its changes slowly and 

 gradually, because of their need of development through many 

 generations. As Darwin said, "it can act only by very short 

 and slow steps." For such a process alone to achieve what has 

 been brought about in the way of organic Evolution would, as 

 I have already intimated, require prodigious periods of time far 

 exceeding anything that geologists would demand, or which 

 physicists are prepared to grant. 



Yet we find Weismann an implicit believer in this one cause. 

 He holds, as we have seen, that "there is no such thing as a 

 'tendency' of the protoplasm to vary," and its probably varied 

 isomeric changes are never once referred to. He appeals to 

 chance variations in the nutrition of his determinants, due to 

 irregularities in the food supply that reaches them within the 

 microscopic granules of which the chromosomes are composed, 

 and regards ' natural selection ' as an all-sufficient cause of organic 

 evolution. I, on the other hand, am impressed with the intrinsic 

 mutability of living matter and its innumerable isomeric states ; 

 and see in this fundamental property of protoplasm a quality 

 which would cause it to vary in form under the influence of its 

 own changing states and ' molecular polarities' ; which would make 

 it sensitive to all external influences, whether acting directly or 

 indirectly ; which would enable it to respond to the effects of use 

 and disuse, and make it prone from slight, often imperceptible, 

 causes to undergo ^wasZ-spontaneous isomeric variations, leading, 

 according as the organism is low or high in the scale of being, 

 either to actual heterogentic transformations or to comparatively 

 slight speciiic ' mutational ' variations. The conjoint co-operation 

 of causes and influences of this kind, acting, not only now, but 

 during all geologic ages, would make it no longer necessary for 

 biologists to require that so many hundreds or even thousands of 

 millions of years should have passed since living things first 

 appeared on the surface of the earth. 



