the
definite influence of opposite conditions. The foregoing facts apparently
give us as good an idea as we are likely for a long time to obtain, how in
many cases external conditions act directly, though not definitely, in
causing modifications of structure.

* * * * *

_Summary._--There can be no doubt, from the facts given in the early part
of this chapter, that extremely slight changes in {290} the conditions of
life sometimes act in a definite manner on our already variable
domesticated productions; and, as the action of changed conditions in
causing general or indefinite variability is accumulative, so it may be
with their definite action. Hence it is possible that great and definite
modifications of structure may result from altered conditions acting during
a long series of generations. In some few instances a marked effect has
been produced quickly on all, or nearly all, the individuals which have
been exposed to some considerable change of climate, food, or other
circumstance. This has occurred, and is now occurring, with European men in
the United States, with European dogs in India, with horses in the Falkland
Islands, apparently with various animals at Angora, with foreign oysters in
the Mediterranean, and with maize grown in Europe from tropical seed. We
have seen that the chemical compounds secreted by plants and the state of
their tissues are readily affected by changed conditions. In some cases a
relation apparently exists between certain characters and certain
conditions, so that if the latter be changed the character is lost--as with
cultivated flowers, with some few culinary plants, with the fruit of the
melon, with fat-tailed sheep, and other sheep having peculiar fleeces.

The production of galls, and the change of plumage in parrots when fed on
peculiar food or when inoculated by the poison of a toad, prove to us what
great and mysterious changes in structure and colour may be the definite
result of chemical changes in the nutrient fluids or tissues.

We have also reason to believe that organic beings in a state of nature may
be modified in various definite ways by the conditions to which they have
been long exposed, as in the case of American trees in comparison with
their representatives in Europe. But in all such cases it is most difficult
to distinguish between the definite results of changed conditions, and the
accumulation through natural selection of servicea