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Show 138 LAWS OF VARIATION. CHAP. v to have increased the size of the eyes ; whereas with all the other inhabitants of the caves, disuse by itself seems to have done its work. It is difficult to imagine conditions of life more similar than deep limestone caverns und~r a nearly. simila.r climate · so that on the common view of the bhnd ani- ' . mals having been separately created for the Amencan and European caverns, close similarity in their organisation and affinities might have been expected; but, as Schiodte and others have remarked, this is not the case, and the cave-insects of the two continents are not more closely allied than might have been anticipated from the general resemblance of the other inhabitants of North America and Europe. On my view we must suppose that American animals, having ordinary powers of vision, slowly migrated by successive 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 gradation of habit ; for, as Schiodte remarks, " animals not far remote from ordinary forms, prepare the transition from light to darkness. Next follow those that are constructed for twilight; and, last of all, those destined for total darkness." By the time that an animal had reached, after numberless generations, the deepest re· cesses, 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 an tennre or pal pi, as a com pensation 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 American cave-animals, as I hear from CHAP. v. ACCLIMATISA. TION. 139 Professor Dana; and some of the European caveinsects are very closely allied to those of the surrounding. country. It would be most difficult to give any rational explanation of the affinities of the blind caveanimals 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 O~d and New Worlds should be closely related, we might expect from the well-known relationship of most of. their other productions. Far from feeling any surprise that some of the cave-animals should be very a~omalous, as Agassiz has remarked in regard to the bhnd fish, the Amblyopsis, and as is the case with the blind Proteus wi~h reference to the reptiles of Europe, I am only surpnsed that more wrecks of ancient life have not been preserved, owing to the less severe competition to which the inhabitants of these dark abodes will probably have been exposed. . Aaalima~isation.-Habit is hereditary with plants, as I~ the period of flowering, in the amount of rain requisite for seeds to germinate, in the time of sleep &c a.nd this leads me to say a few words on acclim~tis~~ tion. As it is extremely common for species of the same genus ~o inhabit very hot and very cold countries, and as I beheve that all the species of the same genus have descended from a single parent, if this view be correct, a?climatisation must be readily effected during long~con~Inued descent. It is notorious that each spec~es Is adapted to the climate of its own home : spemes from an arctic or even from a temperate region can~ot endure a tropical climate, or conversely. So a~ain, many succulent plants cannot endure a damp c~Ima~e. But the degree of adaptation of species to t e ehmates under which they live is often overrated. |