a voyage.[241]

Finally, he adds that these changes cannot be attributed to loss of health
or vigour, "when we reflect how healthy, long-lived, and vigorous many
animals are under captivity, such as parrots, and hawks when used for
hawking, chetahs when used for hunting, and elephants. The reproductive
organs themselves are not diseased; and the diseases from which animals in
menageries usually perish, are not those which in any way affect their
fertility. No domestic animal is more subject to disease than the sheep,
yet it is remarkably prolific.... It would appear that any change in {235}
the habits of life, whatever these habits may be, if great enough, tends to
affect in an inexplicable manner the powers of reproduction."

Such, then, is the singular sensitiveness of the generative system.

As to the means by which that system is affected, we see that a variety of
conditions affect it; but as to the modes in which they act upon it, we
have as yet little if any clue.

We have also seen the singular effects (in tailed Lepidoptera, &c.) of
causes connected with geographical distribution, the mode of action of
which is as yet quite inexplicable; and we have also seen the innate
tendency which there appears to be in certain groups (birds of paradise,
&c.) to develop peculiarities of a special kind.

It is, to say the least, probable that other influences exist, terrestrial
and cosmical, as yet un-noted. The gradually accumulating or diversely
combining actions of all these on highly sensitive structures, which are
themselves possessed of internal responsive powers and tendencies, may well
result in occasional repeated productions of forms harmonious and vigorous,
and differing from the parental forms in proportion to the result of the
combining or conflicting action of all external and internal influences.

If, in the past history of this planet, more causes ever intervened, or
intervened more energetically than at present, we might _a priori_ expect a
richer and more various evolution of forms more radically differing than
any which could be produced under conditions of more perfect equilibrium.
At the same time, if it be true that the last few thousand years have been
a period of remarkable and exceptional uniformity as regards this planet's
astronomical relations, there are then some grounds for thinking that
organic evolution may have been exceptionally depressed during the same
epoch.

Now, as to the fact th