LAWS OF VARIATION 113 



and the similarity which is exhibited in a few forms between the 

 Mammoth Cave (in Kentucky) and the caves in Carniola, other- 

 wise than as a very plain expression of that analogy which subsists 

 generally between the fauna of Europe and of North America." 

 On my view we must suppose that American animals, having in 

 most cases ordinary powers of vision, slowly migrated by suc- 

 cessive generations from the outer world into the deeper and 

 deeper recesses of the Kentucky caves, as did European animals 

 into the caves of Europe. We have some evidence of this grada- 

 tion of habit; for, as Schiodte remarks: "We accordingly look 

 upon the subterranean faunas as small ramifications which have 

 penetrated into the earth from the geographically limited faunas 

 of the adjacent tracts, and which, as they extended themselves into 

 darkness, have been accommodated to surrounding circumstances. 

 Animals not far remote from ordinary forms, prepare the transi- 

 tion from light to darkness. Next follow those that are constructed 

 for twilight; and, last of all, those destined for total darkness, and 

 whose formation is quite peculiar." These remarks of Schiodte's, 

 it should be understood, apply not to the same, but to distinct 

 species. By the time that an animal had reached, after numberless 

 generations, the deepest recesses, disuse will on this view have 

 more or less perfectly obliterated its eyes, and natural selection 

 will often have effected other changes, such as an increase in the 

 length of the antennae or palpi, as a compensation for blindness. 

 Notwithstanding such modifications, we might expect still to see 

 in the cave-animals of America, affinities to the other inhabitants 

 of that continent, and in those of Europe to the inhabitants of the 

 European continent. And this is the case with some of the Ameri- 

 can cave-animals, as I hear from Professor Dana; and some of the 

 European cave-insects are very closely allied to those of the sur- 

 rounding country. It would be difficult to give any rational ex- 

 planation of the affinities of the blind cave-animals to the other 

 inhabitants of the two continents on the ordinary view of their 

 independent creation. That several of the inhabitants of the 

 caves of the Old and New Worlds should be closely related, we 

 might expect from the well-known relationship of most of their 

 other productions. As a blind species of Bathyscia is found in 

 abundance on shady rocks far from caves, the loss of vision in 

 the cave species of this one genus has probably had no relation 

 to its dark habitation; for it is natural that an insect already 

 deprived of vision should readily become adapted to dark caverns. 

 Another blind genus (Anophthalmus) offers this remarkable pe- 

 culiarity, that the species, as Mr. Murray observes, have not as 



