pounded by Andrew Knight,
that this variability may be partly connected with excess of food. It seems
pretty clear that organic beings must be exposed during several generations
to the new conditions of life to cause any appreciable amount of variation;
and that when the organisation has once begun to vary, it generally
continues to vary for many generations. {8} No case is on record of a
variable being ceasing to be variable under cultivation. Our oldest
cultivated plants, such as wheat, still often yield new varieties: our
oldest domesticated animals are still capable of rapid improvement or
modification.

It has been disputed at what period of life the causes of variability,
whatever they may be, generally act; whether during the early or late
period of development of the embryo, or at the instant of conception.
Geoffroy St. Hilaire's experiments show that unnatural treatment of the
embryo causes monstrosities; and monstrosities cannot be separated by any
clear line of distinction from mere variations. But I am strongly inclined
to suspect that the most frequent cause of variability may be attributed to
the male and female reproductive elements having been affected prior to the
act of conception. Several reasons make me believe in this; but the chief
one is the remarkable effect which confinement or cultivation has on the
function of the reproductive system; this system appearing to be far more
susceptible than any other part of the organisation, to the action of any
change in the conditions of life. Nothing is more easy than to tame an
animal, and few things more difficult than to get it to breed freely under
confinement, even in the many cases when the male and female unite. How
many animals there are which will not breed, though living long under not
very close confinement in their native country! This is generally
attributed to vitiated instincts; but how many cultivated plants display
the utmost vigour, and yet rarely or never seed! In some few such cases it
has been discovered that very trifling changes, such as a little more or
less water at some particular period of growth, will determine whether or
not the plant sets a seed. I cannot here enter on the copious details which
I have collected on {9} this curious subject; but to show how singular the
laws are which determine the reproduction of animals under confinement, I
may just mention that carnivorous animals, even from the tropics, breed in
this country pre