ner are highly variable:
specific characters more variable than generic: secondary sexual
characters variable--Species of the same genus vary in an analogous
manner--Reversions to long-lost characters--Summary.

I have hitherto sometimes spoken as if the variations--so common and
multiform with organic beings under domestication, and in a lesser
degree with those under nature--were due to chance. This, of course is
a wholly incorrect expression, but it serves to acknowledge plainly
our ignorance of the cause of each particular variation. Some authors
believe it to be as much the function of the reproductive system to
produce individual differences, or slight deviations of structure, as
to make the child like its parents. But the fact of variations and
monstrosities occurring much more frequently under domestication than
under nature, and the greater variability of species having wide ranges
than of those with restricted ranges, lead to the conclusion that
variability is generally related to the conditions of life to which each
species has been exposed during several successive generations. In the
first chapter I attempted to show that changed conditions act in two
ways, directly on the whole organisation or on certain parts alone, and
indirectly through the reproductive system. In all cases there are two
factors, the nature of the organism, which is much the most important of
the two, and the nature of the conditions. The direct action of changed
conditions leads to definite or indefinite results. In the latter case
the organisation seems to become plastic, and we have much fluctuating
variability. In the former case the nature of the organism is such that
it yields readily, when subjected to certain conditions, and all, or
nearly all, the individuals become modified in the same way.

It is very difficult to decide how far changed conditions, such as of
climate, food, etc., have acted in a definite manner. There is reason
to believe that in the course of time the effects have been greater than
can be proved by clear evidence. But we may safely conclude that the
innumerable complex co-adaptations of structure, which we see throughout
nature between various organic beings, cannot be attributed simply to
such action. In the following cases the conditions seem to have produced
some slight definite effect: E. Forbes asserts that shells at their
southern limit, and when living in shallow water, are more brightly
coloure