onment first lead to alterations in the needs of the organisms,
which, as the result of a kind of conscious effort of will, induce
useful modifications and even the development of new organs. His work
has not exercised any influence on the progress of science: Darwin
himself confessed in regard to Lamarck's work--"I got not a fact or idea
from it." ("Life and Letters", Vol. II. page 215.)

On a mass of incomparably richer and more essential data Darwin
based his view of the descent of organisms and gained for it general
acceptance; as an explanation of modification he elaborated the
ingeniously conceived selection theory. The question of special interest
in this connection, namely what is the importance of the influence
of the environment, Darwin always answered with some hesitation and
caution, indeed with a certain amount of indecision.

The fundamental principle underlying his theory is that of general
variability as a whole, the nature and extent of which, especially
in cultivated organisms, are fully dealt with in his well-known book.
(Darwin, "The variation of Animals and Plants under domestication",
2 vols., edition 1, 1868; edition 2, 1875; popular edition 1905.) In
regard to the question as to the cause of variability Darwin adopts a
consistently mechanical view. He says: "These several considerations
alone render it probable that variability of every kind is directly or
indirectly caused by changed conditions of life. Or, to put the case
under another point of view, if it were possible to expose all the
individuals of a species during many generations to absolutely uniform
conditions of life, there would be no variability." ("The variation of
Animals and Plants" (2nd edition), Vol. II. page 242.) Darwin did not
draw further conclusions from this general principle.

Variations produced in organisms by the environment are distinguished by
Darwin as "the definite" and "the indefinite." (Ibid. II. page 260. See
also "Origin of Species" (6th edition), page 6.) The first occur "when
all or nearly all the offspring of an individual exposed to certain
conditions during several generations are modified in the same manner."
Indefinite variation is much more general and a more important factor in
the production of new species; as a result of this, single individuals
are distinguished from one another by "slight" differences, first in
one then in another character. There may also occur, though this is very
rare, more ma