Heredity and Natural Selection. 307 



The cases where it can be proved to have occurred are 

 not very nnmerons, it is true, bnt there are enough of 

 them to present a serious difficulty. On our view that 

 an external change Avhich acts upon a certain part of the 

 body may cause variation in that particular part, the 

 chances against the parallel modification of allied organ- 

 isms are very greatly diminished, so much so that the 

 occasional occurrence of such modifications might be * 

 expected. If such cases were the rule they would be 

 equally fatal to the theory of natural selection, wlicther 

 our theory of heredity were accepted or not; but the 

 cases are very far from frequent. 

 General and Special Homology : and the Significance 



of Serial Homology, Symmetry and Polymorphism. 



We have, go far, been occupied in studying tlie evi- 

 dence for the law of heredity which is afforded by tlie 

 slight and recently acquired differences between the 

 sexes of the same species, between the young and the 

 adult, between domesticated and wild races, between the 

 hybrid offspring of allied species, between reciprocal hy- 

 brids, etc. 



The bearing of this law upon the more profound prob- 

 lems of morphology has hardly been referred to, for the 

 field which we have examined, although we have passed 

 over it very rapidly, has furnished materialfor a treatise 

 of considerable length. The discussion of the general 

 problems of morphology would require another volume 

 of even greater length, for I believe, and hope to show 

 in another place, that the acceptanceof my view Avill lead 

 to considerable change in our manner of handling these 

 problems ; and will so shift our view as to remodel some 

 of the fundamental principles of the science. 



I believe that it will throw light upon many obscure 

 and perplexing questions, such as the significance of 



