In one case also the
plants themselves became remarkably uniform in height.
The conclusion that the advantages of a cross depend altogether on the
differentiation of the sexual elements, harmonises perfectly with the
fact that an occasional and slight change in the conditions of life is
beneficial to all plants and animals. (12/13. I have given sufficient
evidence on this head in my 'Variation under Domestication' chapter 18
volume 2 2nd edition page 127.) But the offspring from a cross between
organisms which have been exposed to different conditions, profit in an
incomparably higher degree than do young or old beings from a mere
change in the conditions. In this latter case we never see anything like
the effect which generally follows from a cross with another individual,
especially from a cross with a fresh stock. This might, perhaps, have
been expected, for the blending together of the sexual elements of two
differentiated beings will affect the whole constitution at a very early
period of life, whilst the organisation is highly flexible. We have,
moreover, reason to believe that changed conditions generally act
differently on the several parts or organs of the same individual
(12/14. See, for instance, Brackenridge 'Theory of Diathesis' Edinburgh
1869.); and if we may further believe that these now slightly
differentiated parts react on one another, the harmony between the
beneficial effects on the individual due to changed conditions, and
those due to the interaction of differentiated sexual elements, becomes
still closer.
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