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rived forms; and the parent form is the one which happened to be 

 first described in the books. 



Now, I undertake to say that this mode of treating species, though 

 it might have answered fifty years ago, is not satisfactory to-day. The 

 vast majority of naturalists see in resemblance not necessarily identity, 

 but community of origin; they hold that all the species of the same 

 genus are descended from the same progenitor; that intermediate forms 

 between strong varieties descended from a common progenitor tend 

 to become extinct, being absorbed by the stronger forms; that per- 

 manence and breeding true to type are proper characters by which to 

 determine what is a species; that species inheriting nearly the same 

 constitution from a common parent, and exposed to similar influences, 

 naturally tend to produce analogous variations; or that the same spe- 

 cies may occasionally revert to some of the characters of their ancient 

 progenitors. That is the teaching of Darwin, and to refuse to admit 

 it is as if one should refuse to admit the truths taught by Copernicus. 



Every person interested in these questions should read the address 

 by Dr. Asa Gray, " on Sequoia," published in his volume of addresses 

 and essays, entitled "Darwiniana" (Appletons, 1876). In this it is 

 shown how distinct genera of plants are represented by closely 

 allied species, perhaps in very small numbers, in widely distant parts 

 of two continents. In one genus mentioned there are three species 

 only, one of which is found in Florida, one iu California, one in Japan.' 

 In another, four species inhabit Atlantic N. America, California and 

 China. In another case, the single species of a genus, inhabiting a 

 limited district of the Alleghanies, "finds its only and very near rela- 

 tive " in a single species of another genus in Mantchooria. I do not 

 find, by the way, that Dr. Gray gives these related forms any lesser 

 rank than species. On the Hagenian system, in each case the allied 

 species would be looked on as so many derived forms, derived one 

 from the other, and as Dr. Gray shows farther on, this would be im- 

 possible. His explanation is as follows : In remote geological ages 

 the circumpolar regions had a warm climate; vegetation was very like 

 what it is to-day in the temperate zones of both continents. This is 

 proved by the fossil trees and plants which are preserved in the 

 present arctic regions, with characters so distinctly marked as to be 

 capable of identification. The conclusion of Lesquereux, quoted by 

 Dr. Gray, is, that the essential types of our actual flora are marked in 

 the cretaceous period, and have come to us without notable changes 

 through the tertiary formations of our continent. And Dr. Gray says 

 for himself: "I, for one, cannot doubt that the present existing 

 species are lineal successors of those that garnished the earth in the 

 old time before them. The glaciation of the north temperate zone, 

 foUowinp- the warmer period, pushed the species southward, upon 

 widely different longitudes. Hence the diffusion of the same- species 

 over two continents." And he adds, that " this hypothesis supposes 

 a gradual mpdification in different directions, under altering con- 

 ditions, at least to the extent of producing varieties, sub-species and 

 representative species, as they may be variously regarded." 



Now this applies to Isutterflies as well as to plants, and I propose to 



