Sexual Selection.—Not merely do indi viduals struggle for existence, but the males struggle for the 'females, and the most vigorous tend to leave most prog eny. Several weapons, offensive and de fensive, like the cock's spurs, the stag's horns, or the lion's mane, are used in this struggle, and the most useful variations are those which are transmitted.
Laws of Variation.—The same laws appear to have acted in producing the lesser differences between varieties of the same species and the greater differences between species of the same genus. Spe cific characters are more variable than generic, and varietal than either. Rudi mentary organs and secondary sexual characters are variable. Zebra-like stripes on horses, or wood-pigeon's mark ings on fantails, tumblers, etc., may be explained as reversions toward their ancient progenitors.
Geological Succession of Organic Be ings.—The most ancient forms differ widely from those now living, yet fre quently present characters intermediate between groups now widely divergent, and resemble the embryos of the more recent and more highly specialized ani mals belonging to the same classes. Dar vin's belief that the distinctness of birds from all other vertebrates was to be ac counted for by the extinction of a long line of progenitors connecting them with reptiles, was in 1859 a mere assumption; but in 1862 the long-tailed and intensely reptilian bird Archaeopteryx was discov ered, while in 1875 the researches of Marsh brought to light certain cretaceous birds, and the hypothesis of Darwin is thus admirably verified.
Geographical Distribution. — Neither the similarity nor the dissimilarity of the inhabitants of various regions, whether of land or of sea, can be accounted for by identity or differences of climate, or other physical conditions; but both are related in the most striking degree to the absence or presence of barriers to migration be tween those regions.
Morphological Arguments.—The physi ological and distributional lines of argu ment furnished by morphology are mainly four, and are derived from (a) Classification, (b) Homologies, (c) Em bryology, (d) Rudimentary Organs. The great fact of classification is that organic beings, throughout all time, are arranged in groups subordinated under other groups, individuals under varieties, and these again under species; species under genera; genera under sub-families, fam ilies, and orders; and all under a few grand classes. The element of descent is already used in linking all the sexes, ages, forms, and varieties of the same species, widely though these may differ from each other in structure.
The members of the same class, inde pendently of their habits of life, resemble one another in their general plan of or ganization. Thus the hand of man, the digging-paw of the mole, the leg of the horse, the paddle of the porpoise, and the wing of the bat, are all constructed on the same pattern, bone corresponding to bone.
Serial Homology is that unity of type which is found on comparing the differ ent parts and organs in the same individ ual. The complex and varied jaws and legs of a lobster, or the different leaves, sepals, petals, stamens, and pistils of a flower, are all found to be modifications of a simple limb, and a simple leaf-organ respectively. The process of development goes from the general to the special; thus there is generally an advance in organ ization. In peculiar conditions degenera tion may occur. All these facts are ex plained on the principle of successive slight variations not necessarily or gen erally supervening very early in life, and Inherited at a corresponding period; hence it is in the highest degree probable that most embryonic stages show us more or less completely the progenitor of the groups in its adult state; and embryology thus rises greatly in interest.