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01 - Variations Under Domestication 01-01 - Causes of Variability 20 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
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
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
sap
01 - Variations Under Domestication 01-02 - Effects of Habit 20 The great and inherited development of the udders in cows and goats in countries where they are habitually milked, in comparison with these organs in other countries, is probably another instance of the effects of use.
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Not one of our domestic animals can be named which has not in some country drooping ears; and the view which has been suggested that the drooping is due to disuse of the muscles of the ear, from the animals being seldom much alarmed, seems probable.
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01 - Variations Under Domestication 01-03 - correlation of Growth 20 Professor Wyman has recently communicated to me a good illustration of this fact; on asking some farmers in Virginia how it was that all their pigs were black, they informed him that the pigs ate the paint-root (Lachnanthes), which coloured their bones pink, and which caused the hoofs of all but the black varieties to drop off; and one of the "crackers" (i.e. Virginia squatters) added,
"we select the black members of a litter for raising, as they alone have a good chance of living."

Jeffries Wyman
Jeffries Wyman

pig
pig


Hairless dogs have imperfect teeth; long-haired and coarse-haired animals are apt to have, as is asserted, long or many horns;

dog
dog


pigeons with feathered feet have skin between their outer toes; pigeons with short beaks have small feet, and those with long beaks large feet.

Hence if man goes on selecting, and thus augmenting, any peculiarity, he will almost certainly modify unintentionally other parts of the structure, owing to the mysterious laws of correlation.

pigeon
pigeon
01 - Variations Under Domestication 01-04 - Inheritance 20 Any variation which is not inherited is unimportant for us.

But the number and diversity of inheritable deviations of structure, both those of slight and those of considerable physiological importance, are endless. Dr. Prosper Lucas's treatise, in two large volumes, is the fullest and the best on this subject.

No breeder doubts how strong is the tendency to inheritance; that like produces like is his fundamental belief: doubts have been thrown on this principle only by theoretical writers.

When any deviation of structure often appears, and we see it in the father and child, we cannot tell whether it may not be due to the same cause having acted on both; but when amongst individuals, apparently exposed to the same conditions, any very rare deviation, due to some extraordinary combination of circumstances, appears in the parent- say, once amongst several million individuals- and it reappears in the child, the mere doctrine of chances almost compels us to attribute its reappearance to inheritance.

Every one must have heard of cases of albinism, prickly skin, hairy bodies, &c., appearing in several members of the same family.

If strange and rare deviations of structure are really inherited, less strange and commoner deviations may be freely admitted to be inheritable.

Perhaps the correct way of viewing the whole subject would be, to look at the inheritance of every character whatever as the rule, and non-inheritance as the anomaly?
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