which breed freely together,
tending to reproduce their like, and are physiological species. Normally
resembling their parents, the offspring of members of these species are
still liable to vary; and the variation may be perpetuated by selection,
as a race, which race, in many cases, presents all the characteristics
of a morphological species. But it is not as yet proved that a race
ever exhibits, when crossed with another race of the same species, those
phenomena of hybridization which are exhibited by many species when
crossed with other species. On the other hand, not only is it not proved
that all species give rise to hybrids infertile 'inter se', but there
is much reason to believe that, in crossing, species exhibit every
gradation from perfect sterility to perfect fertility.

Such are the most essential characteristics of species. Even were man
not one of them--a member of the same system and subject to the same
laws--the question of their origin, their causal connexion, that is,
with the other phenomena of the universe, must have attracted his
attention, as soon as his intelligence had raised itself above the level
of his daily wants.

Indeed history relates that such was the case, and has embalmed for us
the speculations upon the origin of living beings, which were among the
earliest products of the dawning intellectual activity of man. In those
early days positive knowledge was not to be had, but the craving
after it needed, at all hazards, to be satisfied, and according to the
country, or the turn of thought, of the speculator, the suggestion that
all living things arose from the mud of the Nile, from a primeval
egg, or from some more anthropomorphic agency, afforded a sufficient
resting-place for his curiosity. The myths of Paganism are as dead as
Osiris or Zeus, and the man who should revive them, in opposition to the
knowledge of our time, would be justly laughed to scorn; but the coeval
imaginations current among the rude inhabitants of Palestine, recorded
by writers whose very name and age are admitted by every scholar to be
unknown, have unfortunately not yet shared their fate, but, even at
this day, are regarded by nine-tenths of the civilized world as the
authoritative standard of fact and the criterion of the justice of
scientific conclusions, in all that relates to the origin of things,
and, among them, of species. In this nineteenth century, as at the dawn
of modern physical science, the cosmogony