268 SIGNIFICANCE OF SEXUAL REPRODUCTION [V. 



liarities, for they do not open near the mouth, but upon the 

 forehead, so that the animal can breathe, even in a rough sea, 

 as soon as it comes to the surface. In order to faciHtate rapid 

 movement in water, the whole body has become extended in 

 length, and spindle-shaped, like the body of a fish. The hind 

 limbs are absent in no other mammals, the fish-like Sirenia 

 being alone excepted. In the whales, as in the Sirenia, these 

 appendages have become useless, owing to the powerfully 

 developed tail-fin ; they are now rudimentary and consist of 

 some small bones and muscles deeply buried in the body of the 

 animal, which nevertheless, in certain species, still exhibit the 

 original structure of the hind-limb. The hairy covering of 

 other mammals has also disappeared, its place having been 

 taken by a thick layer of fat beneath the skin, which affords 

 a much better protection against cold. This fatty layer was 

 also necessary in order to diminish the specific gravity of the 

 animal, and to thus render it equal to that of sea-water. In the 

 structure of the skull there are also a number of peculiarities, 

 all of which are directly or indirectly connected with the con- 

 ditions under which these animals live. In the whalebone 

 whales, the enormous size of the face, the immense jaws, and 

 wide mouth are very striking. Can it be suggested that this 

 very characteristic appearance is entirely due to the guidance 

 of some internal transforming force, or to some spontaneous 

 modification of the idioplasm.^ Any such suggestion cannot be 

 accepted, for it is easy to show that all these structural features 

 depend upon adaptation to a peculiar mode of feeding. Func- 

 tional teeth are absent, but rudimentary ones exist in the 

 embryo as relics of an ancestral condition in which these 

 organs were fully developed. Large plates of whalebone with 

 finely divided ends are suspended vertically from the roof of 

 the mouth. These whales feed upon small organisms, about 

 an inch in length, which swim or float upon the water in 

 countless numbers ; and in order that they may subsist upon 

 such minute animals, it is necessary to obtain them in immense 

 numbers. This is achieved by means of the huge mouth which 

 takes in a vast quantity of water at a single mouthful. The 

 water then filters away through the plates of whalebone, while 

 the organisms which form the whale's food remain stranded in 

 the mouth. Is it necessary to add that the internal organs— so 



