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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.
Isidore Geoffroy Saint Hilaire
Isidore Geoffroy Saint Hilaire


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:

sheep
sheep

pig
pig
01 - Variations Under Domestication 01-10 - Methodical and Unconscious Selection 10 At the present time, eminent breeders try by methodical selection, with a distinct object in view, to make a new strain or sub-breed, superior to anything of the kind in the country.

But, for our purpose, a form of Selection, which may be called Unconscious, and which results from every one trying to possess and breed from the best individual animals, is more important.

Thus, a man who intends keeping pointers naturally tries to get as good dogs as he can, and afterwards breeds from his own best dogs, but he has no wish or expectation of permanently altering the breed.

dog
dog


Nevertheless we may infer that this process, continued during centuries, would improve and modify any breed, in the same way as Bakewell, Collins, &c., by this very same process, only carried on more methodically, did greatly modify, even during their lifetimes, the forms and qualities of their cattle.

cattle
cattle


Slow and insensible changes of this kind can never be recognised unless actual measurements or careful drawings of the breeds in question have been made long ago, which may serve for comparison. In some cases, however, unchanged, or but little changed individuals of the same breed exist in less civilised districts, where the breed has been less improved.

There is reason to believe that King Charles's spaniel has been unconsciously modified to a large extent since the time of that monarch.

Some highly competent authorities are convinced that the setter is directly derived from the spaniel, and has probably been slowly altered from it.

spaniel
spaniel

Irish Setter
Irish Setter


It is known that the English pointer has been greatly changed within the last century, and in this case the change has, it is believed, been chiefly effected by crosses with the foxhound; but what concerns us is, that the change has been effected unconsciously and gradually, and yet so effectually, that, though the old Spanish pointer certainly came from Spain, Mr. Borrow has not seen, as I am informed by him, any native dog in Spain like our pointer.

English Pointer
English Pointer

foxhound
foxhound
01 - Variations Under Domestication 01-11 - Unknown Origin of our Domestic Productions 10 A large amount of change, thus slowly and unconsciously accumulated, explains, as I believe, the well-known fact, that in a number of cases we cannot recognise, and therefore do not know, the wild parent-stocks of the plants which have been longest cultivated in our flower and kitchen gardens.

If it has taken centuries or thousands of years to improve or modify most of our plants up to their present standard of usefulness to man, we can understand how it is that neither Australia, the Cape of Good Hope, nor any other region inhabited by quite uncivilised man, has afforded us a single plant worth culture.

It is not that these countries, so rich in species, do not by a strange chance possess the aboriginal stocks of any useful plants, but that the native plants have not been improved by continued selection up to a standard of perfection comparable with that acquired by the plants in countries anciently civilised.

Australia
Australia

Cape of Good Hope
Cape of Good Hope
04 - Natural Selection 04-04 - Its Power at All Ages and on Both Sexes 10 As we see that those variations which, under domestication, appear at any particular period of life, tend to reappear in the offspring at the same period;- for instance, in the shape, size, and flavour of the seeds of the many varieties of our culinary and agricultural plants; in the caterpillar and cocoon stages of the varieties of the silk-worm; in the eggs of poultry, and in the colour of the down of their chickens; in the horns of our sheep and cattle when nearly adult;- so in a state of nature natural selection will be enabled to act on and modify organic beings at any age, by the accumulation of variations profitable at that age, and by their inheritance at a corresponding age.
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silkworm
silkworm

egg
egg

poultry
poultry

chicken
chicken

sheep
sheep

cattle
cattle


If it profit a plant to have its seeds more and more widely disseminated by the wind, I can see no greater difficulty in this being effected through natural selection, than in the cotton-planter increasing and improving by selection the down in the pods on his cotton-trees.

cotton
cotton


Natural selection may modify and adapt the larva of an insect to a score of contingencies, wholly different from those which concern the mature insect; and these modifications may affect, through correlation, the structure of the adult.

larva
larva

insect
insect


So, conversely, modifications in the adult may affect the structure of the larva; but in all cases natural selection will ensure that they shall not be injurious: for if they were so, the species would become extinct.