146 SUMMARY. 



tendency for the blue tint and bars and marks to reappear 

 in the mongrels. I have stated that the most probable 

 hypothesis to account for the reappearance of very ancient 

 characters, is — that there is a tendency in the young of each 

 successive generation to produce the long-lost character, and 

 that this tendency, from unknown causes, sometimes pre- 

 vails. And we have just seen that in several species of the 

 horse genus the stripes are either plainer or appear more 

 commonly in the young than in the old. Call the breeds of 

 pigeons, some of which have bred true for centuries, species ; 

 and how exactly parallel is the case with that of the species 

 of the horse genus ! For myself, I venture confidently to 

 look back thousands on thousands 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 independ- 

 ently 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 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 cosmog- 

 onists, 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 lias 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 differ- 

 ences between species of the same genus. Changed condi- 

 tions generally induce mere ^ .fluctuating variability, but 



