uals proceeding from the same stock are
essentially alike, we infer from their close resemblance that these
similar individuals belong to the same species. That is, we infer it
when the individuals are as much like each other as those are which we
know, or confidently suppose, to have sprung from the same stock.

523. Identity in species is inferred from close similarity in all
essential respects, or whenever the differences, however considerable,
are not known or reasonably supposed to have been originated in the
course of time under changed conditions. No two individuals are exactly
alike; a tendency to variation pervades all living things. In
cultivation, where variations are looked after and cared for, very
striking differences come to light; and if in wild nature they are less
common or less conspicuous, it is partly because they are uncared for.
When such variant forms are pretty well marked they are called

524. =Varieties.= The White Oak, for example, presents two or three
varieties in the shape of the leaves, although they may be all alike
upon each particular tree. The question often arises, and it is often
hard to answer, whether the difference in a particular case is that of a
variety, or is specific. If the former, it may commonly be proved by
finding such intermediate degrees of difference in various individuals
as to show that no clear distinction can be drawn between them; or else
by observing the variety to vary back again in some of its offspring.
The sorts of Apples, Pears, Potatoes, and the like, show that
differences which are permanent in the individual, and continue
unchanged through a long series of generations when propagated by
division (as by offsets, cuttings, grafts, bulbs, tubers, etc.), are not
likely to be reproduced by seed. Still they sometimes are so, and
perhaps always tend in that direction. For the fundamental law in
organic nature is that offspring shall be like parent.

RACES are such strongly marked varieties, capable of coming true to
seed. The different sorts of Wheat, Maize, Peas, Radishes, etc., are
familiar examples. By selecting those individuals of a species which
have developed or inherited any desirable peculiarity, keeping them from
mingling with their less promising brethren, and selecting again the
most promising plants raised from their seeds, the cultivator may in a
few generations render almost any variety transmissible by seed, so long
as it is cared for and kep