Moreover, this belief cannot be extended to the strongly marked
advantages which invariably follow, as far as my experience serves, from
intercrossing the individuals of the same variety or of distinct
varieties, if these have been subjected during some generations to
different conditions.
It is obvious that the exposure of two sets of plants during several
generations to different conditions can lead to no beneficial results,
as far as crossing is concerned, unless their sexual elements are thus
affected. That every organism is acted on to a certain extent by a
change in its environment, will not, I presume, be disputed. It is
hardly necessary to advance evidence on this head; we can perceive the
difference between individual plants of the same species which have
grown in somewhat more shady or sunny, dry or damp places. Plants which
have been propagated for some generations under different climates or at
different seasons of the year transmit different constitutions to their
seedlings. Under such circumstances, the chemical constitution of their
fluids and the nature of their tissues are often modified. (12/8.
Numerous cases together with references are given in my 'Variation under
Domestication' chapter 23 2nd edition volume 2 page 264. With respect to
animals, Mr. Brackenridge 'A Contribution to the Theory of Diathesis'
Edinburgh 1869, has well shown that the different organs of animals are
excited into different degrees of activity by differences of temperature
and food, and become to a certain extent adapted to them.
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