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On the Origin of Species

Laws Of Variation
Effects of changed conditions -- Use and disuse, combined with natural selection; organs
of flight and of vision -- Acclimatisation -- Correlated variation -- Compensation and
economy of growth -- False correlations -- Multiple, rudimentary, and lowly organised
structures variable -- Parts developed in an unusual manner are highly variable: specific
characters more variable than generic: secondary sexual characters variable -- Species of
the same genus vary in an analogous manner -- Reversions to long-lost characters --
Summary.
I have hitherto sometimes spoken as if the variations--so common and multiform with
organic beings under domestication, and in a lesser degree with those under nature--were
due to chance. This, of course is a wholly incorrect expression, but it serves to
acknowledge plainly our ignorance of the cause of each particular variation. Some
authors believe it to be as much the function of the reproductive system to produce
individual differences, or slight deviations of structure, as to make the child like its
parents. But the fact of variations and monstrosities occurring much more frequently
under domestication than under nature, and the greater variability of species having wide
ranges than of those with restricted ranges, lead to the conclusion that variability is
generally related to the conditions of life to which each species has been exposed during
several successive generations. In the first chapter I attempted to show that changed
conditions act in two ways, directly on the whole organisation or on certain parts alone,
and indirectly through the reproductive system. In all cases there are two factors, the
nature of the organism, which is much the most important of the two, and the nature of
the conditions. The direct action of changed conditions leads to definite or indefinite
results. In the latter case the organisation seems to become plastic, and we have much
fluctuating variability. In the former case the nature of the organism is such that it yields
readily, when subjected to certain conditions, and all, or nearly all, the individuals
become modified in the same way.
It is very difficult to decide how far changed conditions, such as of climate, food, etc.,
have acted in a definite manner. There is reason to believe that in the course of time the
effects have been greater than can be proved by clear evidence. But we may safely
conclude that the innumerable complex co-adaptations of structure, which we see
throughout nature between various organic beings, cannot be attributed simply to such
action. In the following cases the conditions seem to have produced some slight definite
effect: E. Forbes asserts that shells at their southern limit, and when living in shallow
water, are more brightly coloured than those of the same species from further north or
from a greater depth; but this certainly does not always hold good. Mr. Gould believes
that birds of the same species are more brightly coloured under a clear atmosphere, than
when living near the coast or on islands; and Wollaston is convinced that residence near
the sea affects the colours of insects. Moquin-Tandon gives a list of plants which, when
growing near the sea-shore, have their leaves in some degree fleshy, though not
elsewhere fleshy. These slightly varying organisms are interesting in as far as they
 
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