12 Heredity. 



adult animal to its proper purpose seemed to be a ni} T 8- 

 tery of the same kind, and many profound thinkers sat- 

 isfied themselves and taught others that this adaptation 

 was not brought about by the laws of matter and by 

 secondary causes; that it must be accepted in itself, 

 without explanation, and that the methods of physical 

 science are here of no use. 



Darwin's work has taught us that this is not true; 

 that in the law of natural selection we have at least a 

 partial explanation of the origin of the adaptation of 

 nature; that while natural selection may not be the ex- 

 clusive means by which they have been produced, it is, 

 so far as it goes, a true scientific explanation, for it even 

 puts it in our power to produce, in domestic animals, 

 similar adaptations to special purposes, by the selection of 

 the fittest variations. 



Darwin, in his first and in all his later books on the 

 subject, pointed out that his discovery did not complete 

 the solution of the problem; that "natural selection is a 

 great but not the exclusive means of modification." The 

 greatest value of his work lies in the proof which he has 

 furnished, that the origin of the structure of animals is 

 not beyond our reach, but that observation and reflec- 

 tion, the means which have unlocked for us so many 

 of the secrets of inorganic nature, are equally useful 

 in this field; that the adaptations of nature may be 

 studied and understood like a problem in astro*nomy or 

 physics. 



The aim of this work is to show that the same thing is 

 true of the problem of heredity. 



We may not be able, as yet, to penetrate its secrets to 

 their inmost depths, but I hope to show that observation 

 and reflection do enable us to discover some of the laws 

 upon which heredity depends, and do furnish us with at 



