CAUSE OF VAEIATIOITS. 147 



This principle of natural selection, which is the basis of the Dar- 

 winian theory, rests, on the one hand, oh the interaction of adaptation 

 and heredity, and on the other, on the struggle for existence which 

 can be shown to occur everywhere in nature. 



In its fundamental idea the natural selection theory is essentially 

 an application of the doctrine of Malthus to plants and animals. 

 Developed simultaneously by Darwin and Wallace/" it received from 

 the former a most comprehensive scientific foundation. 



We must certainly admit that Darwin's selection theory, although 

 supported by what we know of biological processes and of the opera- 

 tion of the laws of nature, is very far from discovering the final 

 causes and physical connection of the phenomena of adaptation and 

 heredity, since it is unable to explain why such or such a variation 

 should appear as the necessary consequence of a change in the vital 

 conditions, and how it is that the manifold and wonderful phenomena 

 of heredity are a function of organised matter. 



It is clearly a great exaggeration when enthusiastic supporters of 

 the Darwinian theory t say that it ranks as equal to the gravitation 

 theory of Newton, because "it is founded upon a single law, a 

 single effective cause, namely, upon the interaction of adaptation and 

 heredity." They overlook the fact that we have here only to do 

 with the proof of a mechanical and causal connection between series 

 of biological phenomena, and not in the remotest degree with a 

 physical explanation. Even if we are justified in connecting the 

 phenomena of adaptation with the processes of nourishment, and in 

 conceiving heredity as a physiological function of the organism, we 

 still stand and regard these phenomena as " the savage who sees 

 a ship for the first time." While the complicated phenomena of 

 heredity J remain completely unintelligible, we are only in a position 

 to explain in general terms certain modifications of organs, on 

 physical grounds, by the altered conditions of metabolism. It is only 

 rarely as in the case of the operation of use and disuse that we 

 are able more directly to relate the development or the atrophy of 

 organs to the increase or decrease in their nutritive activity, i.e., 

 to give a chemico-physical explanation. 



Darwin has been unjustly reproached with having left chance to 



* Compare also A. R. Wallace, " Contributions to the Theory of Natural 

 Selection." 



t Compare E. Haeckel, " Natiivliche Schopfungsgeschichte. 4. Auflage. 

 Berlin, 1873. 



J It is clearly a misuse of the word "Law" to represent the numerous 

 partially opposed and limiting phenomena of heredity as so many " laws of 

 heredity," as E. Haeckel does. 



