134 THE GENESIS OF SPECIES. [CHAP. 



bable that such internal barriers exist in all, although 

 enclosing a much larger field for variation in some cases 

 than in others. Mr. Darwin abundantly demonstrates the 

 variability of dogs, horses, fowls, and pigeons, but he none 

 the less shows clearly the vert/ small extent to which the 

 goose, the peacock, and the guinea-fowl have varied. 1 Mr. 

 Darwin attempts to explain this fact as regards the goose 

 by the animal being valued only for food and feathers, and 

 from no pleasure having been felt in it on other accounts. 

 He adds, however, at the end the striking remark, 2 which 

 concedes the whole position, " but the goose seems to have 

 a singularly inflexible organization.'' This is not the only 

 place in which such expressions are used. He elsewhere 

 makes use of phrases which quite harmonize with the con- 

 ception of a normal specific constancy, but varying greatly 

 and suddenly at intervals. Thus he speaks 8 of a whole 

 organization seeming to have become plastic, and tending to 

 depart from the parental type. That different organisms 

 should have different degrees of variability, is only what 

 might have been expected a priori from the existence of 

 parallel differences in inorganic species, some of these 

 having but a single form, and others being polymorphic. 



To return to the goose, however, it may be remarked 

 that it is at least as probable that its fixity of character is 



stitution of the reproductive system of the species crossed ;" and in vol. ii. 

 at p. 388 may be read the following passage : "In the greater number of 

 cases \ve can only say that the cause of each slight variation and of each 

 monstrosity lies much more in the nature or constitution of the organism 

 than in the nature of the surrounding conditions ; though new and changed 

 conditions certainly play an important part in exciting organic changes of 

 all kinds." 



1 "Animals and Plants under Domestication," vol. i. pp. 289295. 



3 "Origin of Species," 5th edition, 1869, p. 45, 



3 Ibid. p. 13. 



