Page:Popular Science Monthly Volume 7.djvu/668

648 With the more advanced embryos the prominent ear would be conclusive against their avian nature, and the nostrils, where they show, are not those of birds.

We may, then, dismiss from our minds any anxiety as to whether bats are partly birds and partly mammals, and conclude merely that, upon the essential mammalian structure, there have been superinduced features which enable the bat to fly in the air; these, however, no more making it a bird than the form and habit of the whale and manatee render them fishes.

The second question is, whether bats are to be regarded as the progenitors or the descendants of the moles and shrews; or, to put it more accurately (since the idea of derivation does not imply that living species have descended from other living species, but from similar extinct species or from others which combined features since separated in the two forms), is it probable that the existing bats have been produced from original stocks more nearly resembling the moles or the reverse? That the former is the more probable, is indicated upon three grounds:

1. The bat form is peculiar among mammals, and does not, like the Ornithorhynchus and Echidna, manifest any internal structural affinity with birds. There is a much more marked resemblance to the extinct flying reptiles (Pterodactyli), but this is probably one of analogy.

2. The embryo bat resembles the ordinary small mammal; the long fingers, the persistence of the web between them, and its continuation from the border of the body and tail, are features of later appearance.

3. In one embryo (Fig. 9), the thinness and prolongation of the muzzle as compared with the lower jaw may be compared with the elongated snouts of the "star-nosed mole" and the "elephant shrew."

I have never had the opportunity of examining the young of moles or shrews. This would be very desirable, and, one would think, not difficult to accomplish.

Figs. 5 to 11 are intended chiefly to show the gradual development of the limbs, so the other parts are drawn with less detail, and no attempt is made to elucidate the manner of formation of the face from the visceral arches.

The series begins with Fig. 5. Here the body is simply an elongated mass, longer and rounded at the head end, and tapering at the other extremity. It is twisted upon itself, as is often the case with young embryos. The yolk-sack and membranes are not well preserved, and are not shown at all in the figure. This embryo may be regarded as quite small for even a bat. The limbs have not appeared, so the tail does not form a distinct prolongation. (The lower figure is of natural size; the upper is enlarged five diameters.)

In Fig. 6 the arm (ar) and leg (pes) project as little flat pads from the sides of the body. There is no sign of subdivision into fingers and toes, and very little difference between the two limbs. It is worth