Chap. V. 
SUMMAEY. 
167 
centuries, species; and liow exactly parallel is the case 
with that of the species of the horse-genus! For myself, 
I venture confidently to look back thousands on thou¬ 
sands of generations, and I see an animal striped 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 inde¬ 
pendently created, will, I presume, assert that each 
species has been created with a tendency to vary, both 
under nature and under domestication, in this par¬ 
ticular manner, so as often to become striped like 
other species 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 resembling 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 now 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 
differs, more or less, from the same part in the parents. 
But whenever we have the means of instituting a com¬ 
parison, the same laws appear to have acted in pro¬ 
ducing the lesser differences between varieties of the 
same species, and the greater differences between species 
of the same genus. The external conditions of life, as 
