GENERAL PRINCIPLES OF BREEDING. 53 



character, it will require a much greater period 

 of time before breeding in-and-in shall have 

 produced a sufficient degree of unity of organ- 

 ism to interfere with fertility or to cause a loss 

 of vitality than in cases where the stock, to 

 begin with, is of a uniform type, or "purely 

 bred." In such cases, also, there is no effort 

 on the part of the farmer to produce uniformity 

 by selection of individuals for coupling. If 

 there be any selection at all the standard by 

 which it is made is a capricious one, changing 

 from year to year; and it is a well-known fact 

 that in such hands uniformity of type is never 

 reached, neither have any bad effects usually 

 been observed from in-breeding in such cases. 



If the theory above advanced be correct no 

 bad results will necessarily follow from breed- 

 ing in-and-in until uniformity of type, which 

 implies unity of organism, is attained, and this, 

 as we have seen when breeding from a mixed 

 stock, is a very slow process. 



Upon this aspect of the case Herbert Spencer 

 remarks: 



Relations must, on the average of cases, be individuals 

 whose physiological units are more nearly alike than usual. 

 Animals of different varieties must be those whose physio- 

 logical units are more unlike than usual. In the one case the 

 unlikeness of the units may frequently be insufficient to pro- 

 duce fertilization; or if sufficient to produce fertilization not 

 sufficient to produce that active molecular change required 

 for vigorous development. In the other case both fertiliza- 

 tion and vigorous development will be made probable. 



