(3) In cases where the separation has been long continued, and the external conditions are the most diverse in points that involve diversity of adaptation, there we find the most decided divergences in the organic forms. That is, where Separation and Divergent Selection have long acted, the results are found to be the greatest.
The 1st and 3rd of these propositions will probably be disputed by few, if by any. The proof of the 2nd is found wherever a set of closely allied organisms is so distributed over a territory that each species and variety occupies its own narrow district, within which it is shut by barriers that restrain its distribution while each species of the environing types is distributed over the whole territory. The distribution of terrestrial molluscs on the Sandwich Islands presents a great body of facts of this kind.
I will now recapitulate the main doctrines which have been set forth in the foregoing chapter, and then proceed to consider the objections which have been advanced against them.
It must be remembered that by isolation I mean exactly what Mr. Gulick does by "Segregation," and approximately what Professor Weismann does by "Amixia "--i. e. the prevention of intercrossing.
Isolation occurs in very many forms besides the geographical, as will be more fully shown at the end of this chapter; and in all its forms it admits of degrees.
It also occurs in two very different species or kinds--namely, discriminate and indiscriminate. These I have called respectively Homogamy and Apogamy. This all-important distinction has been clearly recognized by Mr. Gulick, as a result of his own thought and observation, independently of anything that I have published upon the subject.
In view of this distinction Isolation takes rank with Heredity and Variability as one of the most fundamental principles of organic evolution. For, if these other two principles be granted, the whole theory of descent resolves itself into an inquiry touching the causes, forms, and degrees of Homogamy.
Save in cases where very large populations are concerned, apogamy must sooner or later give rise _per se_ to homogamy, owing to the Law of Delboeuf, which is the principle that I have called Independent Variability, and Gulick has called Independent Generation. But of course this does not hinder that under apogamy various other causes of homogamy are likely to arise--in particular natural selection.