122 DISTINCT SPECIES PRESENT [CHAP. V. 



sive 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 venture con- 

 fidently 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 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 

 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 living on the sea-shore. 



Summary. Our ignorance of the laws of variation is profound. 

 Not in one case out of a hnndred 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 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 

 to vary in the same manner, and homologous parts tend to cohere. 

 Modifications in hard parts and in external parts sometimes affect 

 softer and internal parts. When one part is largely developed, 

 perhaps it tends to draw nourishment from the adjoining parts ; 

 and every part of the structure which can be saved without detri- 

 ment will be saved. Changes of structure at an early age may 

 affect parts subsequently developed ; and many cases of correlated 



