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Show 180 DIFFICULTIES ON THEORY. CHAP. VI. it leaves the frozen waters, and preys .like other p~led cats on mice and land animals. If a cliffer~nt c~se 1a been taken, and it had been asked how an Jnsectr:orous d quadruped coul possi' bly b ave been converted .I nto. a flying bat, the question would have been far more ~ffi-cult, and I could have given no answer. Yet I think such difficulties have very little weight. Here as on other occasions, I lie under a heavy dis-advant; ge, for out of the many striking cases. which I have collected I can give only one or two Instances of transitional' habits and structures in closely allied species of the same genus ; . and of diversifi~d habits, either constant or occasional, In the same species. And it seems to me that nothing less than a long list of such cases is sufficient to lessen the difficulty in any par-ticular case like that of the bat. Look at the family of squirrels; here we have the finest gradation from animals with ~heir t~ils only slightly flattened, and from oth~rs, as Su J. RI?hards?n has remarked, with the postenor part of their bodies rather wide and with the skin on their flanks rather full, to the so-called flying squirrels; and flying squirrels have their limbs and even the base of the tail united by a broad expanse of skin, which serves as a parachute and allows them to glide through the air to an astonishing distance from tree to tree. We cannot doubt that each sti·ucture is of use to each kind of squirrel in its own eountry, by enabling it to escape birds or beasts of prey, or to collect food more quickly, or, as there is reason to believe, by lessening the danger from occasional falls. But it does not follow from this fact that the structure of each squirrel is the best that it is possible to conceive under all natural conditions. Let the climate and vegetation change, let other competing rodents or new beasts of prey immigrate, or old ones CHAP. VI. TRANSITIONAL HABITS. 181 bec_ome modified, and all analogy would lead us to ?elieve that some at least of the squirrels would decrease 1n numbers ?r becom~ exterminated, unless they also became modified and Improved in structure in a corresponding m~nner. Therefore, I can see no difficulty, more especially under changing conditions of life in the continued preservation of individuals with fuller 'and fuller flank-membranes, each modification being useful, each being propagated, until by the accumulated effects of this process of natural selection, a perfect so-called flying squirrel was produced. Now look at the Galeopithecus or flying lemur which formerly was falsely ranked amongst bats. It has an extremely wide flank-membrane, stretching from the corners of the jaw to the tail, and including the limbs and the elongated fingers : the flank-membrane is, also, furnif:i~ed with an extensor muscle. Although no gr~duated hnks of structure, fitted for gliding through the air, now connect the Galeopithecus with the other Lemuridoo, yet I can see no difficnlty in supposing that such links formerly existed, and that each had been formed ?Y. the sa~e steps as in the case of the less perfectly ghd1ng squurels; and that each grade of structure had been us.eful to i~s possessor. Nor can I see any insuperable difficulty In further believing it possible that the membrane-connected fingers and fore-arm of the Galeopithecus might be greatly leno·thened by natural l . b se ectwn ; and this, as far as the organs of flight are concerned, would convert it into a bat. In bats which have the wing-membrane extended from the top of the shoulder to the tail, including the hind-legs, we perhaps ~e.e traces of an apparatus originally constructed for ghd1ng through the air rather than for flight. If about a dozen genera of birds had become extinct or were unknown, who would have ventured to have |