BATS AND THEIR YOUNG. 6^,7 



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 mat- 

 ter of size, usually regarded as of little moment in zoological discrimi- 

 nation, 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 con- 

 ditions under which it is placed, the first of these conditions being the 

 access of the zoosperms of the male. Now, the eggs of all mammals 

 are small, usually microscopic. The human ovum is about j^ 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 tfian the largest 

 mammalian ovum ; while that of the ostrich or the Epyornis is sim- 

 ply gigantic in comparison. 



Now, I am not aware that the ovum of a bat has ever been exam- 

 ined, 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 em- 

 bryo 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 consider- 

 able 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.' 



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 quadru- 

 peds, are evidently not birds. The tail is too long (for any bird ex- 

 cepting 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. 



1 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. 



