Since, then, questions about the origin of species will be raised, and have been raised,—­and since the theorizings, however different in particulars, all proceed upon the notion that one species of plant or animal is somehow derived from another, that the different sorts which now flourish are lineal (or unlineal) descendants of other and earlier sorts,—­it now concerns us to ask, What are the grounds in Nature, the admitted facts, which suggest hypotheses of derivation, in some shape or other?  Reasons there must be, and plausible ones, for the persistent recurrence of theories upon this genetic basis.  A study of Darwin’s book, and a general glance at the present state of the natural sciences, enable us to gather the following as perhaps the most suggestive and influential.  We can only enumerate them here, without much indication of their particular bearing.  There is,—­

1.  The general fact of variability;—­the patent fact, 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, perhaps 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 appear to vary little, others moderately, others immoderately, to the great bewilderment of systematic botanists and zoologists, and their increasing disagreement as to whether various forms shall be held to be original species or marked 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.