lone. We cannot suppose that minor traits, exemplified among
others by the aesthetic perceptions, can have been evolved by natural
selection. But if there is inheritance of functionally-produced
modifications of structure, evolution of such minor traits is no longer
inexplicable.

* * * * *

Two remarks made by Mr. Darwin have implications from which the same
general conclusion must, I think, be drawn. Speaking of the variability
of animals and plants under domestication, he says:--

"Changes of any kind in the conditions of life, even extremely
slight changes, often suffice to cause variability.... Animals and
plants continue to be variable for an immense period after their
first domestication; ... In the course of time they can be
habituated to certain changes, so as to become less variable; ...
There is good evidence that the power of changed conditions
accumulates; so that two, three, or more generations must be
exposed to new conditions before any effect is visible.... Some
variations are induced by the direct action of the surrounding
conditions on the whole organization, or on certain parts alone,
and other variations are induced indirectly through the
reproductive system being affected in the same manner as is so
common with organic beings when removed from their natural
conditions."--(_Animals and Plants under Domestication_, vol. ii,
270.)

There are to be recognized two modes of this effect produced by changed
conditions on the reproductive system, and consequently on offspring.
Simple arrest of development is one. But beyond the variations of
offspring arising from imperfectly developed reproductive systems in
parents--variations which must be ordinarily in the nature of
imperfections--there are others due to a changed balance of functions
caused by changed conditions. The fact noted by Mr. Darwin in the above
passage, "that the power of changed conditions accumulates; so that two,
three, or more generations must be exposed to new conditions before any
effect is visible," implies that during these generations there is going
on some change of constitution consequent on the changed proportions and
relations of the functions. I will not dwell on the implication, which
seems tolerably clear, that this change must consist of such
modifications of organs as adapt them to their changed functions; and
that