314 



VAEIATION OF PLANTS AND ANIMALS. [Ch. XXXVL 



strange if we are persuaded of the truth of Mr. Darwin's 

 view, that the whole organisation of an animal is so tied to- 

 ether, that when even slight variations occur in any one part 

 other parts usually become modified. 



Among many other illustrations which he gives of this 

 principle, called by him in the ' Origin of Species ' ' correlation 

 of growth; and in his last work ' correlated variability,' he 

 mentions that pigeons with feathered feet have skin between 

 their outer toes, pigeons with short beaks have small feet and 



some 



remar 



entirely white and have blue eyes are generally deaf. One 



months 



began to grow dark-coloured, and then the cat began to 



hear.'^ 



If the sterility of the mule offspring be due, as the same 

 naturahst suggests, to the imperfection of their reproductive 

 organs arising from the blending together of two different 

 structures and constitutions, which causes a disturbance and 

 interferes with the development of the embryo, we might 

 have expected that differences affecting permanently not 

 only the external form and shape, but even the shape of the 

 skull in many vertebrate animals, as well as their instincts 

 and habits, would have been accompanied, when such fixed 

 varieties were crossed, with a disturbance in the reproductive 

 organs and consequent sterility in the hybrids. 



At the same time we must remember that the 

 changes in races have been brought about by selecti'on, and 



greate st 



tive 



s never oeen tne oDject of man to modify the reproduc- 

 organs with a view of producing two races mutually 



manner 



make 



to proceed. Moreove 



seen how possible it is to alter the foliage of plants without 

 their seeds varying, or to change their seeds, fruit, or flowers 

 without the character of the root or leaves being affected. 

 It is in fact established, in spite of correlation,' that we may 

 cause ^ some organs to be greatly modified, while another 

 to which we have not directed our attention may continue 



* Dr. Sichet, cited by Darwin ' On Variation,' p. 329. 



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