LAWS OF VARIATION 229 



like a zebra, but perhaps otherwise very differently 

 constructed, the common parent of our domestic horse 

 (whether or not it be descended from one or more wild 

 stocks), of the ass, the hemionus, quagga, and zebra. 



He who believes that each equine species was in- 

 dependently created, will, I presume, assert that each 

 species has been created with a tendency to vary, both 

 under nature and under domestication, in this particular 

 manner, so as often to become striped like the other spe- 

 cies of the genus; and that each has been created with 

 a strong tendency, when crossed with species inhabiting 

 distant quarters of the world, to produce hybrids resem- 

 blmg in their stripes, not their own parents, but other 

 species of the genus. To admit this view is, as it seems 

 to me, to reject a real for an unreal, or at least for an 

 unknown, cause. It makes the works of God a mere 

 mockery and deception; I would almost as soon believe 

 with the old and ignorant cosmogonists, that fossil shells 

 had never lived, but had been created in stone so as to 

 mock the shells living on the sea-shore. 



Summary 



Our ignorance of the laws of variation is profound. 

 Not in one case out of a hundred can we pretend to 

 assign any reason why this or that part has varied. But 

 whenever we have the means of instituting a comparison, 

 the same laws appear to have acted in producing the 

 lesser differences between varieties of the same species, 

 and the greater differences between species of the same 

 genus. Changed conditions generally induce mere fluc- 

 tuating variability, but sometimes they cause direct and 

 definite effects; and these may become strongly marked 



