| It has been disputed at what period of time 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 functions of the
reproductive system; this system appearing to be far more susceptible
than any other part of the organization, 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 found out 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 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 pretty freely under confinement, with the
exception of the plantigrades or bear family; whereas, carnivorous
birds, with the rarest exceptions, hardly ever lay fertile eggs. Many
exotic plants have pollen utterly worthless, in the same exact condition
as in the most sterile hybrids. When, on the one hand, we see
domesticated animals and plants, though often weak and sickly, yet
breeding quite freely under confinement; and when, on the other hand, we
see individuals, though taken young from a state of nature, perfectly
tamed, long-lived, and healthy (of which I could give numerous
instances), yet having their reproductive system so seriously affected
by unperceived causes as to fail in acting, we need not be surprised at
this system, when it does act under confinement, acting not quite
regularly, and producing offspring not perfectly like their parents or
variable.
Chapter 1-Variation Under Domestication
Charles Darwin
The Origin of Species |
