01 - Variations Under Domestication

01-01 - Causes of Variability

As far as I am able to judge, after long attending to the subject, the conditions of life appear to act in two ways,- directly on the whole organization or on certain parts alone, and indirectly by affecting the reproductive system.

With respect to the direct action, we must bear in mind that in every case, as Professor Weismann has lately insisted, and as I have incidentally shown in my work on Variation under Domestication, there are two factors: namely, the nature of the organism, and the nature of the conditions.

August Weismann

The former seems to be much the more important; for nearly similar variations sometimes arise under, as far as we can judge, dissimilar conditions; and, on the other hand, dissimilar variations arise under conditions which appear to be nearly uniform.

The effects on the offspring are either definite or indefinite.

They may be considered as definite when all or nearly all the offspring of individuals exposed to certain conditions during several generations are modified in the same manner.

It is extremely difficult to come to any conclusion in regard to the extent of the changes which have been thus definitely induced.

There can, however, be little doubt about many slight changes,- such as size from the amount of food, colour from the nature of the food, thickness of the skin and hair from climate, &c.

Each of the endless variations which we see in the plumage of our fowls must have had some efficient cause; and if the same cause were to act uniformly during a long series of generations on. many individuals, all probably would be modified in the same manner.

fowl

Such facts as the complex and extraordinary out-growths which variably follow from the insertion of a minute drop of poison by a gall-producing insect, show us what singular modifications might result in the case of plants from a chemical change in the nature of the sap. Indefinite variability is a much more common result of changed conditions than definite variability, and has probably played a more important part in the formation of our domestic races.

sap

01 - Variations Under Domestication

01-01 - Causes of Variability

With respect to what I have called the indirect action of changed conditions, namely, through the reproductive system of being affected, we may infer that variability is thus induced, partly from the fact of this system being extremely sensitive to any change in the conditions, and partly from the similarity, as Kreuter and others have remarked, between the variability which follows from the crossing of distinct species, and that which may be observed with plants and animals when reared under new or unnatural conditions.

Many facts clearly show how eminently susceptible the reproductive system is to very slight changes in the surrounding conditions.

Nothing is more easy than to tame an animal, and few things more difficult than to get it to breed freely under confinement, even when the male and female unite.

How many animals there are which will not breed, though kept in an almost free state in their native country!

This is generally, but erroneously, attributed to vitiated instincts.

Many cultivated plants display the utmost vigour, and yet rarely or never seed! In some few cases it has been discovered that a very trifling change, such as a little more or less water at some particular period of growth, will determine whether or not a plant will produce seeds.


I cannot here give the details which I have collected and elsewhere published on this curious subject; but to show how singular the laws are which determine the reproduction of animals under confinement, I may 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, which seldom produce young; whereas carnivorous birds, with the rarest exceptions, hardly ever lay fertile eggs.


Many exotic plants have pollen utterly worthless, in the same condition as in the most sterile hybrids.

When, on the one hand, we see domesticated animals and plants, though often weak and sickly, breeding 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 to act, we need not be surprised at this system, when it does act under confinement, acting irregularly, and producing offspring somewhat unlike their parents.

pollen

01 - Variations Under Domestication

01-02 - Effects of Habit

Changed habits produce an inherited effect, as in the period of the flowering of plants when transported from one climate to another. With animals the increased use or disuse of parts has had a more marked influence; thus I find in the domestic duck that the bones of the wing weigh less and the bones of the leg more, in proportion to the whole skeleton, than do the same bones in the wild-duck;

duck

and this change may be safely attributed to the domestic duck flying much less, and walking more, than its wild parents.