Lines of Variation 



Darwin's main ideas on variation separately, 

 and to consider to what extent they seem to 

 require modification in the light of later research. 



Firstly, Darwin believed that variations arise 

 in what appears to be a haphazard manner, that 

 they occur in all directions, and seem to be 

 governed by the same laws as chance. It is 

 our belief that we are now in a position to make 

 more definite statements regarding variation than 

 Darwin was able to. 



Biologists can now assert definitely that varia- 

 tions do not always occur equally in all directions. 

 The results of many years of the efforts of practi- 

 cal breeders demonstrate this. These men have 

 not been able to produce a green horse, a pigeon 

 with alternate black and white feathers in the tail, 

 or a cat with a trunk, for the simple reason that 

 the organisms upon which they operated do not 

 happen to have varied in the required direction. 

 It may perhaps be objected that breeders have 

 no desire to produce such forms; had they wished 

 to do so, they would probably have succeeded. 

 To this objection we may reply that they have 

 not managed to produce many organisms, which 

 would be highly desirable from a breeder's point 

 of view, as, for example, a blue rose, hens that lay 

 brown eggs but do not become broody at certain 

 seasons of the year, or a cat that cannot scratch. 



As Mivart well says, on page 1 18 of his Genesis 

 of Species, "Not only does it appear that there are 



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