When a variation is of the slightest use to a being, we cannot tell how much of it to attribute to the accumulative action of Natural Selection, and how much to the conditions of life. The same author and some botanists have further remarked that multiple parts are also very liable to variation in structure. The most distinct breeds of pigeons, in countries most widely apart, present sub varieties with reversed feathers on the head and feathers on the feet characters not possessed by the aboriginal rock-pigeon; these then are analogous variations in two or more distinct races. The frequent presence of fourteen or even sixteen tail-feathers in the pouter, may be considered as a variation representing the normal structure of another race, the fantail. Changes of structure at an early age will generally affect parts subsequently developed; and there are very many other correlations of growth, the nature of which we are utterly unable to understand.