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Ch. XXX YII. 1 



BEEEDINa IN AXD IN INJUEIOUS. 



321 



sometimes 



generations in succession, and every trace of the foreign ad- 

 mixture will be lost. The mutual absorption in this manner 

 of the European and negro races the one into the other, by 

 a certain number of intermarriages with one of the two stocks 



has been frequently verified. The efficacy of the principle 

 above adverted to, in causing species to breed true for ages, 

 and checking lawless divergence, in spite of the numerous 

 varieties which occur in every generation, is obvious ; the 

 only difficulty is to conceive how, if there be such proneness 

 in each aberrant form to merge into the normal type, a new 



It would 



■manent 



seem to require prolonged isolation under altered conditions, 

 such as may occur in different parts of the same continent, or 

 still more frequently in different islands of the same archi- 

 pelago. But we have yet to learn what degree of divergence 



•om 



before a decided disinclination to breed together will arise 

 and how much farther this must be carried before the ofl^ 

 spring of the cross, if produced, will be sterile. 

 • Breeding in and in injurious.~It has already been stated 

 that certain domestic races prefer breeding with their own 

 kind ; on the other hand, it is well ascertained that too much, 

 breeding in and in has an injurious effect. 



The half- wild cattle which have been kept for four or five 



British parks, as in those of Lord 

 Duke of Hamilton, where the total 

 number varies from sixty to eighty, are relatively far less 

 fertile than the enormous herds of half-wild cattle in South 

 America. But even in the latter case it is believed that the 



centuries or more 

 Tankerville and the 



necessary to prevent degeneration 

 The decrease in bnlk from anmVnf.i 



from 



IS 



m 



size and fertility.^- 



alluded to must, says , Darwin, have been prodigious, as ae- 

 cordmg to Eutmejer they are the descendants of the igantic 



JJos primigemus. The Chilliuf 



this 



ham cattle are white, but 



is partly due to selection, as dark-coloured calves 



* Darwin ■ On Variation/ chap, xvii., who cites Azars 



are 



VOL. II. 



T 



V.r. ' 



.1 - 



