But variation in an upward direction also, when it has once been set a-going, may in many cases go on unchecked until limits are set to it by personal selection, when the excess of the organ would disturb the harmony of the parts, or in any other way lessen the individual's chances of survival in the struggle for existence. This is proved especially by the phenomena of artificial selection, for almost all the parts of fowls and pigeons have been caused to vary to excess by breeding, and must thus have been, so to speak, capable of unlimited increase; and yet, as we have seen, personal selection cannot directly cause progress in any direction of variation; it can only secure a free course by excluding from breeding the bearers of variations with an opposite tendency. The beards of hens, the tail-feathers of the long-tailed domestic cocks, the long and short, straight and curved bills of pigeons, the enormously long ruffled feathers of the Jacobin, the multiplication of the tail-feathers in the fan-tail, and innumerable other breed-characters of these playthings of the breeder, prove that when variation-tendencies of any part are once present, that is, when they have arisen through germinal selection, they apparently go on unchecked until their further development would permanently and irretrievably destroy the harmony of the parts. As soon as this is threatened the breed loses its power of survival, and Darwin in his time cited the case of many extremely short-billed breeds of pigeon, which require the aid of the breeder before they can emerge from the hard-shelled egg, because their short and soft bills no longer allow them to break their way out. Here the correlation between the hardness of the egg-shell and that of the pigeon's bill has been disturbed, and the breed can now only be kept in existence by artificial aid. There must be a possibility of something similar occurring in natural conditions, and when it does the species concerned must die out. But in the majority of cases the self-regulation which is afforded by personal selection will be enough to force back an organ which is in the act of increasing out of due proportion to within its proper limits. The bearers of such excessively increased determinants succumb in the struggle for existence, and the determinants are thus removed from the genealogical lineage of the species.