LAWS OF VARIATION 133 



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 prevails. 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 ven- 

 ture confidently to look back thousands on thousands of genera- 

 tions, and I see an animal striped like a zebra, but perhaps other- 

 wise 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 independently 

 created, will, I presume, assert that each species has been created 

 with a tendency to vary, both under nature and under domestica- 

 tion, 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 dis- 

 tant 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 un- 

 real, 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 eld and ignorant cosmogonists, that fossil shells had never 

 lived, but had been created in stone so as to mock the shells liv- 

 ing on the seashore. 



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 fluctuating 

 variability, but sometimes they cause direct and definite effects; 

 and these may become strongly marked in the course of time, 

 though we have not sufficient evidence on this head. Habit in 

 producing constitutional peculiarities, and use in strengthening 

 and disuse in weakening and diminishing organs, appear in many 

 cases to have been potent in their effects. Homologous parts tend 



