entary organs have passed in being reduced to their
present condition; but we so incessantly see in species of the same
group the finest gradations between an organ in a rudimentary and
perfect state, that we are led to believe that the passage must have
been extremely gradual. It may be doubted whether a change of structure
so abrupt as the sudden loss of an organ would ever be of service to a
species in a state of nature; for the conditions to which all organisms
are closely adapted usually change very slowly. Even if an organ did
suddenly disappear in some one individual by an arrest of development,
intercrossing with the other individuals of the same species would
cause it to reappear in a more or less perfect manner, so that its
final reduction could only be effected by the slow process of continued
disuse or natural selection. It is much more probable that, from
changed habits of life, organs first become of less and less use, and
ultimately superfluous; or their place may be supplied by some other
organ; and then disuse, acting on the offspring through inheritance at
corresponding periods of life, would go on reducing the organ; but as
most organs could be of no use at an early embryonic period, they would
not be affected by disuse; consequently they would be preserved at this
stage of growth, and would remain as rudiments. In addition to the
effects of disuse, the principle of economy of growth, already alluded
to in this chapter, would lead to the still further reduction of all
superfluous parts. With respect to the final and total suppression or
abortion of any organ, another and distinct principle, which will be
discussed in the chapter on pangenesis, probably takes a share in the
work.

With animals and plants reared by man there is no severe or recurrent
struggle for existence, and the principle of economy will not come into
action. So far, indeed, is this from being the case, that in some
instances organs, which are naturally rudimentary in the
parent-species, become partially redeveloped in the domesticated
descendants. Thus cows, like most other ruminants, properly have four
active and two rudimentary mammae; but in our domesticated animals, the
latter occasionally become considerably developed and yield milk. The
atrophied mammae, which, in male domesticated animals, incl