and the incorporate agencies must either
bring power up by resource and invention, or must pull desire back by
eating less, both as individuals, and as the race, that is to say, by
breeding less freely; for breeding is an assimilation of outside matter
so closely akin to feeding, that it is only the feeding of the race, as
against that of the individual.

I do not think the reader will find any clearer manner of picturing to
himself the development of organism than by keeping the normal growth of
wealth continually in his mind. He will find few of the phenomena of
organic development which have not their counterpart in the acquisition
of wealth. Thus a too sudden acquisition, owing to accidental and
external circumstances and due to no internal source of energy, will be
commonly lost in the next few generations. So a sudden sport due to a
lucky accident of soil will not generally be perpetuated if the
offspring plant be restored to its normal soil. Again, if the advance in
power carry power suddenly far beyond any past desire, or be far greater
than any past-remembered advance of power beyond desire--then desire
will not come up level easily, but only with difficulty and all manner
of extravagance, such as is likely to destroy the power itself. Demand
and Supply are also good illustrations.

But to return to Dr. Darwin.

"When we revolve in our minds," he writes, "first the great changes
which we see naturally produced in animals after their nativity, as in
the production of the butterfly with painted wings from the crawling
caterpillar; or of the respiring frog from the subnatant tadpole; from
the boy to the bearded man, from the infant girl to the woman,--in both
which cases mutilation will prevent due development.

"Secondly, when we think over the great changes introduced into various
animals by artificial or accidental cultivation, as in horses, which we
have exercised for the different purposes of strength or swiftness, in
carrying burthens or in running races, or in dogs which have been
cultivated for strength and courage, as the bull-dog; or for acuteness
of his sense of smell, as the hound or spaniel; or for the swiftness of
his foot, as the greyhound; or for his swimming in the water or for
drawing snow sledges, as the rough-haired dogs of the north; or, lastly,
as a play dog for children, as the lapdog; with the changes of the forms
of the cattle which have been domesticated from the greatest antiquity,
a