you take exactly the same elements, Ass and Horse, but you combine the
sexes in a different manner, and the result is modified accordingly.
You have in this case, however, a result which is not general and
universal--there is usually an important preponderance, but not always
on the same side.

Here, then, is one intelligible, and, perhaps, necessary cause of
variation: the fact, that there are two sexes sharing in the production
of the offspring, and that the share taken by each is different and
variable, not only for each combination, but also for different members
of the same family.

Secondly, there is a variation, to a certain extent--though, in all
probability, the influence of this cause has been very much
exaggerated--but there is no doubt that variation is produced, to a
certain extent, by what are commonly known as external conditions,--such
as temperature, food, warmth, and moisture. In the long run, every
variation depends, in some sense, upon external conditions, seeing that
everything has a cause of its own. I use the term "external
conditions" now in the sense in which it is ordinarily employed: certain
it is, that external conditions have a definite effect. You may take a
plant which has single flowers, and by dealing with the soil, and
nourishment, and so on, you may by-and-by convert single flowers into
double flowers, and make thorns shoot out into branches. You may
thicken or make various modifications in the shape of the fruit. In
animals, too, you may produce analogous changes in this way, as in the
case of that deep bronze colour which persons rarely lose after having
passed any length of time in tropical countries. You may also alter
the development of the muscles very much, by dint of training; all the
world knows that exercise has a great effect in this way; we always