decreases the length of the intestines. Continued disuse, on the other hand, weakens and diminishes all parts of the organisation. Animals which during many generations have taken but little exercise, have their lungs reduced in size, and as a consequence the bony fabric of the chest, and the whole form of the body, become modified. With our anciently domesticated birds, the wings have been little used, and they are slightly reduced; with their decrease, the crest of the sternum, the scapulæ, coracoids, and furcula, have all been reduced.
With domesticated animals, the reduction of a part from disuse is never carried so far that a mere rudiment is left, but we have good reason to believe that this has often occurred under nature. The cause of this difference probably is that with domestic animals not only sufficient time has not been granted for so profound a change, but that, from not being exposed to a severe struggle for life, the principle of the economy of organisation does not come into action. On the contrary, we sometimes see that structures which are rudimentary in the parent-species become partially redeveloped in their domesticated progeny. When rudiments are formed or left under domestication, they are the result of a sudden arrest of development, and not of long-continued disuse with the absorption of all superfluous parts; nevertheless they are of interest, as showing that rudiments are the relics of organs once perfectly developed.
Corporeal, periodical, and mental habits, though the latter have been almost passed over in this work, become changed under domestication, and the changes are often inherited. Such changed habits in any organic being, especially when living a free life, would often lead to the augmented or diminished use of various organs, and consequently to their modification. From long-continued habit, and more especially from the occasional birth of individuals with a slightly different constitution, domestic animals and cultivated plants become to a certain extent acclimatised, or adapted to a climate different from that proper to the parent-species.
Through the principle of correlated variability, when one part varies other parts vary,—either simultaneously, or one after the other. Thus an organ modified during an early embryonic period affects other parts subsequently developed. When an
organ, such as the beak, increases or decreases in length, adjoining or correlated parts, as the tongue and the orifice of the nostrils, tend to vary in the same manner. When the whole body increases or decreases in size, various parts become modified; thus with pigeons the ribs increase or decrease in number and breadth. Homologous parts, which are identical during their early development and are exposed to similar conditions, tend to vary in the same or in some connected manner,—as in the case of the right and left sides of the body, of the front and hind limbs, and even of the head and limbs. So it is with the organs of sight and hearing; for instance, white cats with blue eyes are almost always deaf. There is a manifest relation throughout the body between the skin and its various appendages of hair, feathers, hoofs, horns, and teeth. In Paraguay, horses with curly hair have hoofs like those of a mule; the wool and the horns of sheep vary together; hairless dogs are deficient in their teeth; men with redundant hair have abnormal teeth, either deficient or in excess. Birds with long wing-feathers usually have long tail-feathers. When long feathers grow from the outside of the legs and toes of pigeons, the two outer toes are connected by membrane; for the whole leg tends to assume the structure of the wing. There is a manifest relation between a crest of feathers on the head and a marvellous amount of change in the skull of various fowls; and in a lesser degree, between the greatly elongated, lopping ears of rabbits and the structure of their skulls. With plants, the leaves, various parts of the flower, and the fruit, often vary together in a correlated manner.
In some cases we find correlation without being able even to conjecture what is the nature of the connexion, as with various correlated monstrosities and diseases. This is likewise the case with the colour of the adult pigeon, in connexion with the presence of down on the young bird. Numerous curious instances have been given of peculiarities of constitution, in correlation with colour, as shown by the immunity of individuals of some one colour from certain diseases, from the attacks of parasites, and from the action of certain vegetable poisons.
Correlation is an important subject; for with species, and in a lesser degree with domestic races, we continually find that
certain parts have been greatly modified to serve some useful purpose; but we almost invariably find that other parts have likewise been more or less modified, without our being able to discover any advantage in the change. No doubt great caution is necessary in coming to this conclusion, for it is difficult to overrate our ignorance on the use of various parts of the organisation; but from what we have now seen, we may believe that many modifications are of no direct service, having arisen in correlation with other and useful changes.
Homologous parts during their early development evince an affinity for each other,—that is, they tend to cohere and fuse together much more readily than other parts. This tendency to fusion explains a multitude of normal structures. Multiple and homologous organs are especially liable to vary in number and probably in form. As the supply of organised matter is not unlimited, the principle of compensation sometimes comes into action; so that, when one part is greatly developed, adjoining parts or functions are apt to be reduced; but this principle is probably of much less importance than the more general one of the economy of growth. Through mere mechanical pressure hard parts occasionally affect soft adjoining parts. With plants the position of the flowers on the axis, and of the seeds in the capsule, sometimes leads, through a freer flow of sap, to changes of structure; but these changes are often due to reversion. Modifications, in whatever manner caused, will be to a certain extent regulated by that co-ordinating power or nisus formativus, which is in fact a remnant of one of the forms of reproduction, displayed by many lowly organised beings in their power of fissiparous generation and budding. Finally, the effects of the laws, which directly or indirectly govern variability, may be largely influenced by man's selection, and will so far be determined by natural selection that changes advantageous to any race will be favoured and disadvantageous changes checked.
Domestic races descended from the same species, or from two or more allied species, are liable to revert to characters derived from their common progenitor, and, as they have much in common in their constitutions, they are also liable under changed conditions to vary in the same manner; from these