" I think this holds true to a certain extent with our domestic
productions: if nourishment flows to one part or organ in excess, it
rarely flows, at least in excess, to another part; thus it is difficult to
get a cow to give much milk and to fatten readily. The same varieties of
the cabbage do not yield abundant and nutritious foliage and a copious
supply of oil-bearing seeds. When the seeds in our fruits become
atrophied, the fruit itself gains largely in size and quality. In our
poultry, a large tuft of feathers on the head is generally accompanied by a
diminished comb, and a large beard by diminished wattles. With species in
a state of nature it can hardly be maintained that the law is of universal
application; but many good observers, more especially botanists, believe in
its truth. I will not, however, here give any instances, for I see hardly
any way of distinguishing between the effects, on the one hand, of a part
being largely developed through natural selection and another and adjoining
part being reduced by the same process or by disuse, and, on the other
hand, the actual withdrawal of nutriment from one part owing to the excess
of growth in another and adjoining part.
I suspect, also, that some of the cases of compensation which have been
advanced, and likewise some other facts, may be merged under a more general
principle, namely, that natural selection is continually trying to
economise in every part of the organisation.
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