itions not natural to them, they are extremely liable to vary,
which is due, as I believe, to their reproductive systems having been
specially affected, though in a lesser degree than when sterility
ensues. So it is with hybrids, for hybrids in successive generations are
eminently liable to vary, as every experimentalist has observed.

Thus we see that when organic beings are placed under new and unnatural
conditions, and when hybrids are produced by the unnatural crossing of
two species, the reproductive system, independently of the general state
of health, is affected by sterility in a very similar manner. In the
one case, the conditions of life have been disturbed, though often in so
slight a degree as to be inappreciable by us; in the other case, or
that of hybrids, the external conditions have remained the same, but
the organisation has been disturbed by two different structures and
constitutions having been blended into one. For it is scarcely possible
that two organisations should be compounded into one, without some
disturbance occurring in the development, or periodical action, or
mutual relation of the different parts and organs one to another, or to
the conditions of life. When hybrids are able to breed inter se, they
transmit to their offspring from generation to generation the same
compounded organisation, and hence we need not be surprised that their
sterility, though in some degree variable, rarely diminishes.

It must, however, be confessed that we cannot understand, excepting
on vague hypotheses, several facts with respect to the sterility of
hybrids; for instance, the unequal fertility of hybrids produced from
reciprocal crosses; or the increased sterility in those hybrids which
occasionally and exceptionally resemble closely either pure parent. Nor
do I pretend that the foregoing remarks go to the root of the matter:
no explanation is offered why an organism, when placed under unnatural
conditions, is rendered sterile. All that I have attempted to show,
is that in two cases, in some respects allied, sterility is the common
result,--in the one case from the conditions of life having been
disturbed, in the other case from the organisation having been disturbed
by two organisations having been compounded into one.

It may seem fanciful, but I suspect that a similar parallelism extends
to an allied yet very different class of facts. It is an old and almost
universal belief, founded, I think, on a conside