M Database Inspector (cheetah)
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|OriginOfSpecies - 475 Rows|
|01 - Variations Under Domestication||01-03 - correlation of Growth||10||
Many laws regulate variation, some few of which can be dimly seen, and will hereafter be briefly discussed.
I will here only allude to what may be called correlated variation. Important changes in the embryo or larva will probably entail changes in the mature animal.
In monstrosities, the correlations between quite distinct parts are very curious; and many instances are given in Isidore Geoffroy St-Hilaire's great work on this subject.
Breeders believe that long limbs are almost always accompanied by an elongated head.
Some instances of correlation are quite whimsical: thus cats which are entirely white and have blue eyes are generally deaf;
but it has been lately stated by Mr. Tait that this is confined to the males.
Colour and constitutional peculiarities go together, of which many remarkable cases could be given amongst animals and plants.
From facts collected by Heusinger, it appears that white sheep and pigs are injured by certain plants, whilst dark-coloured individuals escape:
|01 - Variations Under Domestication||01-04 - Inheritance||10||
The results of the various, unknown, or but dimly understood laws of variation are infinitely complex and diversified.
It is well worth while carefully to study the several treatises on some of our old cultivated plants, as on the hyacinth, potato, even the dahlia, &c. and it is really surprising to note the endless points of structure and constitution in which the varieties and sub-varieties differ slightly from each other.
The whole organisation seems to have become plastic, and departs in a slight degree from that of the parental type.
|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?
|01 - Variations Under Domestication||01-04 - Inheritance||30||
The laws governing inheritance are for the most part unknown.
No one can say why the same peculiarity in different individuals of the same species, or in different species, is sometimes inherited and sometimes not so; why the child often reverts in certain characters to its grandfather or grandmother or more remote ancestor; why a peculiarity is often transmitted from one sex to both sexes, or to one sex alone, more commonly but not
exclusively to the like sex.
It is a fact of some importance to us, that peculiarities appearing in the males of our domestic breeds are often transmitted, either exclusively or in a much greater degree, to the males alone.
A much more important rule, which I think may be trusted, is that, at whatever period of life a peculiarity first appears, it tends to reappear in the offspring at a corresponding age, though sometimes earlier. In many cases this could not be otherwise;
thus the inherited peculiarities in the horns of cattle could appear only in the offspring when nearly mature; peculiarities in the silkworm are known to appear at the corresponding caterpillar or cocoon stage.
But hereditary diseases and some other facts make me believe that the rule has a wider extension, and that, when there is no apparent reason why a peculiarity should appear at any particular age, yet that it does tend to appear in the offspring at the same period at which it first appeared in the parent.
I believe this rule to be of the highest importance in explaining the laws of embryology.
These remarks are of course confined to the first appearance of the peculiarity, and not to the primary cause which may have acted on the ovules or on the male element; in nearly the same manner as the increased length of the horns in the offspring from a short-horned cow by a long-horned bull, though appearing late in life, is clearly due to the male element.