Page:Popular Science Monthly Volume 9.djvu/564

538 Every class is again subdivided into certain subordinate groups, termed orders.

Each order is composed of families, each family of genera, and each genus of its component kinds or "species."

Now, the bat, as already said, belongs to man's own class, possessing as it does all the characters which distinguish that class from the other classes of vertebrate animals.

Man's own class, Mammalia, is divisible into some dozen orders, and all the bats form one such order (Cheiroptera), into which no animal but a bat is admitted. The characters of this order are the possession of a truly flying membrane, sustained by very elongated fingers; and the bat is capable of being very shortly defined, namely, as a truly flying mammal.

Bats present no real resemblance whatever to birds, but are, of course, much more like ourselves (who are their class-fellows) than they are like any bird.

Similarly, in spite of this analogical relation of bats to those extinct reptiles, the pterodactyls, these creatures have no true affinity. Pterodactyls are aërial modifications of the Reptilian type, just as bats are aërial modifications of the Mammalian type. We may say, in a rough and general way, as pterodactyls are to reptiles, so are bats to mammals.

Before concluding we may now glance at the question of the genesis or origin of bats. To those who accept the doctrine of Evolution—as I myself do—there can be no question but that bats did arise by natural generation from some anterior beasts which were not bats. But at what period and from what progenitors? these are questions which it is quite impossible to answer at present. As has been said, there are certain cases in which we may imagine now existing more highly specialized and differentiated forms were developed from anterior less highly specialized and differentiated ones. We may do so, e. g., as regards the horse and the ox. But we cannot do so as regards the bat, because up to the present time no fossil remains whatever have been found which connect bats with other creatures. Moreover, the development of the bat's wing, difficult as it is to conceive upon any view of evolution, seems to me to be especially difficult as the mere result of the survival of the fittest, when we consider the origin of the initial stages of the organ. The nearest existing relatives of the bats which are not bats are perhaps the little shrew-mice belonging to the order Insectivora. Some of these are aquatic; and it is conceivable, though there is no fragment of evidence in favor of it, that some ancestral aquatic form may have developed long fingers and webs like those of the flying-frog. This speculation does not, however, commend itself to my mind as a satisfactory one; and though, doubtless, could we see all the extinct forms of life which have existed during the secondary period, we should find some