130 THE EVOLUTION THEORY 



of the relevant determinants towards the minus direction may thus 

 occur, sometimes sooner, sometimes later ; but when once they have 

 started they proceed irresistibly, though with exceeding slowness. 



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 rufiled feathers of the Jacobin, the mul- 

 tiplication 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. 



Having now established the fact that determinants can continue 

 their direction of variation without limit because of internal, that is 

 intra-germinal, reasons, we have come nearer an understanding of 



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