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When we compare the individuals of the same variety or sub-variety of our older cultivated plants and animals, one of the first points which strikes us is, that they generally differ more from each other than do the individuals of any one species or variety in a state of nature.
this variability may be partly connected with excess of food.
there are two factors: namely, the nature of the organism and the nature of the conditions.
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.
At long intervals of time, out of millions of individuals reared in the same country and fed on nearly the same food, deviations of structure so strongly pronounced as to deserve to be called monstrosities arise;
considered as the in definite effects of the conditions of life on each individual organism,
While individual difference in the response to condition maybe important, it requires the condition to take effect or to trigger the individual difference in response
variability is thus induced, partly from the fact of this system being extremely sensitive to any change in the conditions,
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
clearly see that the nature of the conditions is of subordinate importance in comparison with the nature of the organism in determining each particular form of variation; perhaps of not more importance than the nature of the spark, by which a mass of combustible matter is ignited, has in determining the nature of the flames.
but when among individuals, apparently exposed to the same conditions, any very rare deviation, due to some extraordinary combination of circumstances, appears in the parent—say, once among several million individuals—and it reappears in the child, the mere doctrine of chances almost compels us to attribute its reappearance to inheritance.
If it could be shown that our domestic varieties manifested a strong tendency to reversion—that
I also crossed a barb with a spot, which is a white bird with a red tail and red spot on the forehead, and which notoriously breeds very true; the mongrels were dusky and mottled. I then crossed one of the mongrel barb-fantails with a mongrel barb-spot, and they produced a bird of as beautiful a blue colour, with the white loins, double black wing-bar, and barred and white-edged tail-feathers, as any wild rock-pigeon!
No one supposes that our choicest productions have been produced by a single variation from the aboriginal stock.
but, as a general rule, it cannot be doubted that the continued selection of slight variations, either in the leaves, the flowers, or the fruit, will produce races differing from each other chiefly in these characters.
It has consisted in always cultivating the best known variety, sowing its seeds, and, when a slightly better variety chanced to appear, selecting it, and so onwards.
Thus also with ants, the several worker-castes are generally quite distinct; but in some cases, as we shall hereafter see, the castes are connected together by finely graduated varieties.
has marked for me 182 British plants, which are generally considered as varieties, but which have all been ranked by botanists as species;
Mr. Walsh ranks the forms which it may be supposed would freely intercross, as varieties;
The term variety, again, in comparison with mere individual differences, is also applied arbitrarily, for convenience sake.
more especially to the members of the same genus or class having nearly similar habits of life.
Where many species of a genus have been formed through variation, circumstances have been favourable for variation; and hence we might expect that the circumstances would generally still be favourable to variation.
We have seen that there is no infallible criterion by which to distinguish species and well-marked varieties;
the species of the larger genera resemble varieties, more than do the species of the smaller genera.
So that the acknowledged varieties have very nearly the same restricted average range, as have the closely allied forms,
for we should have to give it some advantage over a different set of competitors or enemies.
how infinitely complex and close-fitting are the mutual relations of all organic beings to each other and to their physical conditions of life; and consequently what infinitely varied diversities of structure might be of use to each being under changing conditions of life.
the ordinary belief that the amount of possible variation is a strictly limited quantity, is likewise a simple assumption.
owing to the law of correlation, when one part varies and the variations are accumulated through natural selection, other modifications, often of the most unexpected nature, will ensue.
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.
there must be much fortuitous destruction, which can have little or no influence on the course of natural selection.
In such cases, if the varying individual did not actually transmit to its offspring its newly-acquired character, it would undoubtedly transmit to them, as long as the existing conditions remained the same, a still stronger tendency to vary in the same manner.
the tendency to vary in the same manner
a cross between different varieties, or between individuals of the same variety but of another strain, gives vigour and fertility to the offspring; and on the other hand, that CLOSE interbreeding diminishes vigour and fertility;
it appears that with animals and plants an occasional intercross between distinct individuals is a very general, if not universal, law of nature.
These anomalous forms may be called living fossils; they have endured to the present day, from having inhabited a confined area, and from having been exposed to less varied, and therefore less severe, competition.
I conclude that for terrestrial productions a large continental area, which has undergone many oscillations of level, will have been the most favourable for the production of many new forms of life, fitted to endure for a long time and to spread widely.
the islands were reconverted into a continental area, there will again have been very severe competition; the most favoured or improved varieties will have been enabled to spread; there will have been much extinction of the less improved forms, and the relative proportional numbers of the various inhabitants of the reunited continent will again have been changed;
Owing to the high geometrical rate of increase of all organic beings, each area is already fully stocked with inhabitants, and it follows from this, that as the favoured forms increase in number, so, generally, will the less favoured decrease and become rare.
And we have seen in the chapter on the Struggle for Existence that it is the most closely-allied forms,—varieties of the same species, and species of the same genus or related genera,—which, from having nearly the same structure, constitution and habits, generally come into the severest competition with each other.
Nevertheless according to my view, varieties are species in the process of formation,
it is far more probable that each form remains for long periods unaltered, and then again undergoes modification.
Within the same large group, the later and more highly perfected sub-groups, from branching out and seizing on many new places in the polity of nature, will constantly tend to supplant and destroy the earlier and less improved sub-groups.
Although few of the most ancient species have left modified descendants, yet, at remote geological periods, the earth may have been almost as well peopled with species of many genera, families, orders and classes, as at the present day.
It might be thought that the amount of change which the various parts and organs pass through in their development from embryo to maturity would suffice as a standard of comparison;
the conditions of life to which each species has been exposed during several successive generations.
but likewise to include natural selection, for the conditions determine whether this or that variety shall survive.
Hence adaptation to any special climate may be looked at as a quality readily grafted on an innate wide flexibility of constitution, common to most animals.
Hence modifications of structure, viewed by systematists as of high value, may be wholly due to the laws of variation and correlation, without being, as far as we can judge, of the slightest service to the species.
We may often falsely attribute to correlated variation structures which are common to whole groups of species, and which in truth are simply due to inheritance;
the amount of variation in the individuals of the same species is so great that it is no exaggeration to state that the varieties of the same species differ more from each other in the characters derived from these important organs, than do the species belonging to other distinct genera.
