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 expect to find the
arm of a blacksmith hard and wiry, and possessing a large development
of the brachial muscles. No doubt training, which is one of the forms
of external conditions, converts what are originally only instructions,
teachings, into habits, or, in other words, into organizations, to a
great extent; but this second cause of variation cannot be considered
to be by any means a large one. The third cause that I have to mention,
however, is a very extensive one. It is one that, for want of a better
name, has been called "spontaneous variation;" which means that when
we do not know anything about the cause of phenomena, we call it
spontaneous. In the orderly chain of causes and effects in this world,
there are very few things of which it can be said with truth that they
are spontaneous. Certainly not in these physical matters,--in
these there is nothing of the kind,--everything depends on previous
conditions. But when we cannot trace the cause of phenomena, we call
them spontaneous.

Of these variations, multitudinous as they are, but little is known with
perfect accuracy. I will mention to you some two or three cases, because
they are very remarkable in themselves, and also because I shall want to
use them afterwards. Reaumur, a famous French naturalist, a great
many years ago, in an essay which he wrote upon the art of