154 THE POPULAR SCIENCE MONTHLY. 



otherwise inexplicaMe facts. These examples of incongruous 

 thoughts I give to prepare the way ; and will now go on to ex- 

 amine the chief propositions which the quoted passage contains. 



The Duke of Argyll says that " heredity is the central idea 

 of natural selection." Now it would, I think, be concluded that 

 those who possess the central idea of a thing have some con- 

 sciousness of the thing. Yet men have possessed the idea of 

 heredity for any number of generations and have been quite un- 

 conscious of natural selection. Clearly the statement is mis- 

 leading. It might just as truly be said that the occurrence of 

 structural variations in organisms is the central idea of natural 

 selection. And it might just as truly be said that the action of 

 external agencies in killing some individuals and fostering 

 others is the central idea of natural selection, No such asser- 

 tions are correct. The process has three factors — heredity, vari- 

 ation and external action — any one of which being absent the 

 process ceases. The conception contains three corresponding 

 ideas, and if any one be struck out the conception cannot be 

 framed. No one of them is the central idea, but they are co- 

 essential ideas. 



From the erroneous belief that " heredity is the central idea 

 of natural selection " the Duke of Argyle draws the conclusion, 

 consequently erroneous, that " natural seclection includes and cov- 

 ers all the causes which can possibly operate through inherit- 

 ance." Had he considered the cases which, in the Principles of 

 Biology, I have cited to illustrate the inheritance of functionally- 

 produced modifications, he would have seen that his inference is 

 far from correct. I have instanced the decrease of the jaw 

 among civilised men as a change of structure which cannot have 

 been produced by the inheritance of spontaneous, or fortuitous, 

 variations. That changes of structure arising from such varia- 

 tions may be maintained and increased in successive generations, 

 it is needful that the individuals in whom they occur shall de- 

 rive from them advantages in the struggle for existence — ad- 

 vantages, too, sufficiently great to aid their survival and mul- 

 tiplication in considerable degrees. But a decrease of jaw, re- 

 ducing its weight by even an ounce (which would be a large 

 variation), cannot, by either smaller weight carried or smaller 

 nutrition required, have appreciably advantaged any person in 

 the battle of life. Even supposing such diminution of jaw to be 

 beneficial (and in the resulting decay of teeth it entails great 

 evils), the benefit can hardly have been such as to increase the 

 relative multiplication of families in which it occurred genera- 

 tion after generation. Unless it has done this, however, de- 

 creased size of the jaw cannot have been produced by the nat- 

 ural selection of favorable variations. How can it then have 



