THE METHOD OF THEIR INVESTIGATION. 3 Il 
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, lam convinced, throw light on many problems, correcting 
many partial and incomplete theories. We may also hope that a 
caréful 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? 
