The main argument of Darwin's chief work runs somewhat after the following fashion[2]:-- Variation, to a greater or less degree, is a common and well-known fact in nature. More especially, animals and plants under domestication tend to vary from one another far more than do the individuals of any one species in the wild state. Rabbits in a warren are all alike in shape, size, colour, and features: rabbits in a hutch vary indefinitely in the hue of their fur, the length of their ears, the character of their coat, and half a dozen other minor particulars, well known to the observant souls of boys and fanciers. This great variability, though partly perhaps referable to excess of food, is probably due on the whole to their having been raised under conditions of life not so uniform as, and somewhat different from, those to which the parent species is commonly exposed in a state of nature. In other words, variability is one result of altered and more varied surrounding circumstances. Again, this variability is usually indefinite. You cannot say what direction it will take, or to what particular results it is likely in any special instance to lead. Marked differences sometimes occur even between the young of the same litter, or between the seedlings sown from the same capsule. As a rule, the variations exhibit themselves in connection with sexual reproduction; but sometimes, as in the case of 'sporting plants,' a new bud suddenly produces leaves or flowers of a different character from the rest of those on the self-same stem, thus showing that the tendency to vary is inherent, as it were, in the organism itself. Upon this fundamental fact of the existence in nature of numerous and indefinite variations, the whole theory of natural selection is ultimately built up. In illustrating by example the immense variability of domesticated creatures, Darwin lays great stress upon the case of pigeons, with which he was familiar from his long experience as a breeder and fancier in his own home at Down. Naturalists are almost universally of opinion that all the breeds of domestic pigeons, from the carrier to the tumbler, from the runt to the fantail, are alike descended from the wild rock pigeon of the European coasts. The immense amount of variation which this original species has undergone in domestication may be seen by comparing the numberless breeds of pigeon now exhibited at all our poultry shows with one another.