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 this 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. If under
changed conditions of life a structure before useful becomes less
useful, any diminution, however slight, in its development, will be
seized on by natural selection, for it will profit the individual not
to have its nutriment wasted in building up an useless structure.
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