796 LIFE : OUTLINES OF GENERAL BIOLOGY 



breathing while the milk is being forced down its throat. The 

 temporarily continuous passage from the nostrils onwards allows 

 the air to pass down the windpipe to the lungs, but prevents the 

 milk going down the wrong way. 



Another very interesting adaptation is that the teat or mamma 

 swells inside the young marsupial's mouth, so that the very helpless 

 creature is not likely to lose its hold. But the most important point 

 is the general one, that the short gestation is correlated with a very 

 half-finished state at the time of birth, a drawback which is met 

 by the development of the marsupium or pouch. Thus the hint that 

 we get from the remarkable state of affairs in marsupials is that a 

 prolongation of the period of gestation is likely to be associated 

 with the better equipment of the newly born yoang one. It would 

 be easy to find a female Kangaroo about the same size as a Shetland 

 pony mare; but the former carries its young for 39 days before 

 birth, and the latter for eleven months. Our proposition is that the 

 extension of the duration of ante-natal life has been one of the 

 conditions of the precocity of the newly born foal — an idea that was 

 first promulgated by Robert Chambers, the pre-Darwinian evolu- 

 tionist, who wrote The Vestiges of Creation (1844). 



Length of Gestation. — ^There is evidently some puzzle in the 

 great variety in the length of the ante-natal period in different 

 mammalian types. "Why does the seal take eleven or twelve 

 months for gestation when a large dog requires only nine weeks?" 

 (F. H. A. Marshall, Physiology of Reproduction (1910).) Why does 

 such a highly specialised mammal as a bat have a short gestation — 

 in some species seven weeks — while in another climax of bpecialisa- 

 tion, the elephant, it is twenty months, or even over two years? 

 Why should a tigress carry her unborn young for twenty-two weeks, 

 while the period for the nearly related lioness is sixteen ? 



Peculiar Cases. — In some mammals there are altogether peculiar 

 features which must be kept by themselves, and not allowed to 

 complicate the general problem. Thus in some of the European bats 

 the pairing and insemination normally occur in autumn, whereas 

 the ovulation and fertilisation are delayed tiU the following spring. 

 It seems that the spermatozoa remain stored in the genital duct of 

 the female throughout the whole period of hibernation. In some 

 other mammals, where the fertilisation immediately follows the 

 insemination and the development begins at once, just as is usual, 

 there is a strange arrest of development for a prolonged period. 

 Thus Marshall notes that while roedeer in Germany pair in autumn, 

 the embryo does not develop beyond the segmentation stage till 

 the following spring. There is often a long arrest in the development 

 of the badger. 



The General Theory. — Many variations in the length of the 

 ante-natal period in mammals have been recorded, especially 



