That species differ not only in single characters, but in many, Mr. Darwin accounts for by reference to unknown laws of the correlation of organs—laws, however, which, although unknown, we know to exist, so that a modification of one organ is attended with modification of other organs, as is exemplified in our domestic breeds.
in further support of the theory of natural selection, the fact is insisted upon, "that it is the common. the widely diffused, and widely ranging species, belonging to the larger genera within each class, which vary most." That the several subordinate groups rn auy class of creatures " cannot be ranked in a single file, but seem rather to be clus tered round points, and these round other points, and so on in almost endless cycles," Mr. Darwin thinks incapable of explanation, except on the supposition of community of origin and natural selection. He points also to the analogous manner in which spe cies of the same genus vary, as corroborative of his views. He accounts for the absence or rarity of transitional varieties by supposing the predominant forms to have taken possession of their districts, while these were in process of being stocked; and that these districts, differing much in their natural characters, the forms originating in the com partitively unextensive intermediate tracts, have not been able to contend against them, and have become extinct. He points out the possibility that areas now continuous may not have been so during a icing period, and that species may have been formed while they were broken up into islands. But this remains a chief difficulty of his theory.
He aseribes'to natural selection the results which Lamarck ascribed to use and disuse in the development and atrophy of organs; and thinks it not impossible that the flying squirrels may thus have had a common origin with the true squirrels, and the galeop, thecus with the lemurs, although he admits that we have no graduated links of structure them together. "Nor can I see any insuperable difficulty," he says, " in I art her believing that the membrane-connected lingers and forearm of the caleopithecus might be greatly lengthened by natural selection; and this, as far as the organs of flight are concerned, would convert it into a bat." Like Lain:trek and the authol' of the Ves tiges, Mr. Darwin rests not a little on the unity of type throughout whole classes of
creatures, and the hoinologies of parts very different from each other, as in the limbed structure of the vertebrate generally, and even the articulations of the limbs. He endeavors to trace the eye from its simplest to its most perfect form, and shows how gradual are the transitions found on coihparison of existing creatures, from the one to the other. lie even further, and says: " Several facts make me suspect that nerves sensitive to touch may be rendered sensitive to light, and likewise to those coarser vibra tions of the'air which produce sound." He dwells at great length on the 'subject of hybridism, and the general sterility of hybrids, endeavoring to show that it presents no insuperable objection to the theory of a gradual modification of species, their sterility being incidental on other differences, and sterility occurring, as he labors to prove, when varieties are crossed, as well as in the hybrids of distinct species. The difficulties presented by geology he obviates very much by insisting on the imperfection of the geological record. He does not adopt the view of the author of the Vestiges, that the geological record exhibits to ms a succession of animals corresponding in their progressive development with the fetal development of the mammalian embryo. But he founds an argument on the many connecting links in the general system of nature which fossils supply when compared with existing species. And he endeavors to show that his theory is perfectly consistent with the known facts of the.geographieal distribution of species, and in particular with the remarkable facts of the peculiarity of the fauna and flora of some of the lonely oceanic islands. mid of the frequent occurrence of the same species both in cold regions comparatively near the pole. and on mountains far remote from each other in lower latitudes; referring the lat class of factsnth former geologic periode, when the continental areas were not the .eatee as now, or when the prevailing climatic conditions were very different. And he finds support for his views in the correspondence, without identity, of the floats and faunas of the northern parts of America and of the Old World.