THE METHOD OF THEIR INVESTIGATION. 1 1 



make a full classification of the different forms of interaction that 

 tend to modify the species. A systematic and thorough use of this 

 method will, I am convinced, throw light on many problems, correcting 

 many partial and incomplete theories. We may also hope that a 

 careful examination of the different forms of interaction will, in some 

 degree, lessen the danger of attributing exclusively to one form of 

 interaction results that are really due to several forms. And having 

 discovered that similar results are produced by different forms of 

 action, we are next led to seek for the underlying principle in which 

 they agree. 



4. Natural and Sexual Selection not the only Factors producing 

 Transformation. 



The relation of the species found in any one of the Galapagos 

 Islands to those found on other islands of the same archipelago, and, 

 still further, their relation to the species in South America, suggested 

 to Darwin the idea that they had arisen through the modification of 

 South American species. This idea he elaborated, supplementing 

 and supporting it by attributing the transformation of species to two 

 chief causes — natural and sexual selection. That these two factors 

 must be effective in producing permanent transformation was argued 

 from the effect of artificial selection in producing divergent races of 

 domestic plants and animals, and from the observed fact that in 

 many cases natural varieties and species present degrees of diver- 

 gence corresponding to the time during which they must have been 

 exposed to different environments. These principles have thrown a 

 flood of light on differences between the sexes of the same species, and 

 on those differences of species by which they are adapted to their 

 different environments; but do they show that there can be no 

 divergence in the isolated portions of a species exposed to the same 

 environment, or that all the divergences that arise in portions ex- 

 posed to different environments are adaptations to the environment? 

 Are all the diversities of sexual selection by which different portions 

 of a species are differently modified due to differences in the environ- 

 ments of these portions? If not, we have a cause of divergence that 

 does not depend on exposure to different environments. Moreover, 

 if we assume, as most do, that the differences in sexual selection in the 

 separate portions of a species are due to differences in the sexual 

 instincts of the portions, the question arises as to how we are to 

 explain the divergence in the sexual instincts of individuals exposed 

 to the same environment. Is it not apparent that in the facts 

 brought forward for the proof of this principle of transformation other 

 principles are involved? 



