For the solution of this, as of most such inquiries, we must appeal to embryology, to the study of the development of animals, and of the resemblances between the earlier stages of some and the later stages of others.
Our first object is to confirm the conclusion that bats are mammals rather than birds. And here, strangely enough, we find that the matter of size, usually regarded as of little moment in zoological discrimination, becomes of primary importance. All animals, mammals as well as birds, are formed from eggs. An egg, or ovum, is a cell with special endowments, and capable of availing itself of the peculiar conditions under which it is placed, the first of these conditions being the access of the zoösperms of the male. Now, the eggs of all mammals are small, usually microscopic. The human ovum is about 1⁄120 of an inch in diameter.
Therefore, although the yolk or essential part of the egg of a humming-bird may be pretty small, it is far larger than the largest mammalian ovum; while that of the ostrich or the Epyornis is simply gigantic in comparison.
Now, I am not aware that the ovum of a bat has ever been examined, but there can be doubt of its minuteness as compared with that of any known bird. Fig. 5 shows, of its natural size, the earliest embryo of a bat I have ever heard of. Its length as it lies is much less than that of a humming-bird's egg. Moreover, since the young bird is developed upon the yolk, and the latter remains of considerable size until very near the period of hatching, and since the yolk of our little bat either has been already absorbed or is too minute for detection, it may be considered that it was much smaller than that of birds.
Finally, the simple fact that the little bat was taken out of the mother already somewhat advanced in development, is clear proof that it is not a bird.[1]
Aside from the absence of yolk, the form of the smallest embryo, above figured, might not determine its mammalian nature; but the remaining figures, however little some of them may resemble quadrupeds, are evidently not birds. The tail is too long (for any bird excepting the Archeopteryx); the muzzle is rounded, the feet have five divisions more or less marked, while no bird has more than four toes; and, although the hands may in some cases resemble a bird's wing, yet here too are five fingers, and the wing is evidently an expansion of the hand itself by the elongation and separation of the fingers, rather than a slender hand with feathers attached to the hinder border as with birds.
- ↑ There are no known birds which normally produce living young; but I have a chicken-like body nearly three inches long, which was developed within the hen. It was shown at the meeting of the American Association for the Advancement of Science, this year. Its exact nature can only be learned after full examination of the structure.
