1.  The general fact of variability, and the general tendency of the variety to propagate its like—­the patent facts that all species vary more or less; that domesticated plants and animals, being in conditions favorable to the production and preservation of varieties, are apt to vary widely; and that, by interbreeding, any variety may be fixed into a race, that is, into a variety which comes true from seed.  Many such races, it is allowed, differ from each other in structure and appearance as widely as do many admitted species; and it is practically very difficult, even impossible, to draw a clear line between races and species.  Witness the human races, for instance.  Wild species also vary, perhaps about as widely as those of domestication, though in different ways.  Some of them apparently vary little, others moderately, others immoderately, to the great bewilderment of systematic botanists and zoologists, and increasing disagreement as to whether various forms shall be held to be original species or strong varieties.  Moreover, the degree to which the descendants of the same stock, varying in different directions, may at length diverge, is unknown.  All we know is, that varieties are themselves variable, and that very diverse forms have been educed from one stock.

2.  Species of the same genus are not distinguished from each other by equal amounts of difference.  There is diversity in this respect analogous to that of the varieties of a polymorphous species, some of them slight, others extreme.  And in large genera the unequal resemblance shows itself in the clustering of the species around several types or central species, like satellites around their respective planets.  Obviously suggestive this of the hypothesis that they were satellites, not thrown off by revolution, like the moons of Jupiter, Saturn, and our own solitary moon, but gradually and peacefully detached by divergent variation.  That such closely-related species may be only varieties of higher grade, earlier origin, or more favored evolution, is not a very violent supposition.  Anyhow, it was a supposition sure to be made.

3.  The actual geographical distribution of species upon the earth’s surface tends to suggest the same notion.  For, as a general thing, all or most of the species of a peculiar genus or other type are grouped in the same country, or occupy continuous, proximate, or accessible areas.  So well does this rule hold, so general is the implication that kindred species are or were associated geographically, that most trustworthy naturalists, quite free from hypotheses of transmutation, are constantly inferring former geographical continuity between parts of the world now widely disjoined, in order to account thereby for certain generic similarities among their inhabitants; just as philologists infer former connection of races, and a parent language, to account for generic similarities among existing languages.  Yet no scientific explanation has been offered to account for the geographical association of kindred species, except the hypothesis of a common origin.