Mr. Wallace does not, in my judgement, give sufficient grounds for rejecting the proposition which he indicates as the main point of Mr. Gulick's valuable essay on _Divergent Evolution through Cumulative Segregation_. Mr. Gulick's idea is that ... no two portions of a species possess exactly the same average character, and the initial differences will, if the individuals of the two groups are kept from intercrossing, assert themselves continuously by heredity in such a way as to ensure an increasing divergence of the forms belonging to the two groups, amounting to what is recognized as specific distinction. Mr. Gulick's idea is simply the recognition of a permanence or persistency in heredity, which, _caeteris paribus_, gives a twist or direction to the variations of the descendants of one individual as compared with the descendants of another[8]. [8] _Nature_, Oct. 10, 1889, p. 568. Now we have seen that "Mr. Gulick's idea," although independently conceived by him, had been several times propounded before; and it is partly implicated in more than one passage of the _Origin of Species_, where free intercrossing, or the _absence_ of isolation, is alluded to as maintaining the _constancy_ of a specific type[9]. Moreover, it is still more fully recognized in the last edition of the _Variation of Animals and Plants_, where a paragraph is added for the purpose of sanctioning the principle in the imperfect form that it was stated by Weismann[10]. Nevertheless, to Mr. Gulick belongs the credit, not only of having been the first to conceive (though the last to publish) the "idea" in question, and of having stated it with greater fullness than anybody else; but still more of having verified its importance as a factor of organic evolution. [9] e. g. p. 81. [10] See Chapter xxiii. vol. ii. p. 262. (Edition of 1888.) For, in point of fact, Mr. Gulick was led to his recognition of the principle in question, not by any deductive reasoning from general principles, but by his own particular and detailed observations of the land mollusca of the Sandwich Islands. Here there are an immense number of varieties belonging to several genera; but every variety is restricted, not merely to the same island, but actually to the same valley. Moreover, on tracing this fauna from valley to valley, it is apparent that a slight variation in the occupants of valley 2 as compared with those of the adjacent valley 1, becomes more pronounced in the next--valley 3, still more so in 4, &c., &c. Thus it was possible, as Mr. Gulick says, roughly to estimate the amount of divergence between the occupants of any two given valleys by measuring the number of miles between them.