314 THE EVOLUTION THEORY 



the ear have also disappeared, for the hearing organ of this aquatic type 

 is no longer suited for receiving the sound-waves through an air- 

 containing trumpet, but receives them by a shorter route from the 

 surrounding water, directly through the hones of the skull. Remark- 

 able changes in the respiratory and circulatory organs make prolonged 

 submersion possible, and the displacement of the external nares from 

 the snout to the forehead enables the animal to draw breath when it 

 comes up from the depths to a frequenth^ stormy surface. It would 

 take a long time to enumerate all that can be recognized as adaptive 

 in these remarkable aquatic mammals to a life in what to their 

 ancestors would have been a strange and hostile element. Let us 

 study particularly the case of the whalebone whales, for instance the 

 Greenland whale, and we are at once struck I)}' the enormous size of 

 the head, which makes up about a third of the whole body (Fig. 130). 

 Can this, which has such an important effect in determining the 

 whole type of animal, be an outcome of some internal power of 

 development ? By no means ! It is rather an adaptation to the 

 mode of nutrition peculiar to this swimming mammal, for it does not, 

 like dolphins and toothed whales, feed on large fishes and Cephalopods, 

 Ijut on minute delicate molluscs — Pteropods and pelagic Gastropods, 

 on Salpa?, and the like, which often cover the surface of the 

 Arctic Ocean in endless shoals, sometimes extending for many miles. 

 To enable the whale to sustain life on such minute morsels it was 

 necessary that it should be able to swallow enormous quantities ; 

 teeth were therefore useless, and they have become rudimentary, 

 and can only be demonstrated in the embryo as rudiments (dental 

 germs) in the jaw ; but in place of these there hang from the roof 

 of the mouth-cavit}^ great plates of ' whalebone,' a quite peculiar 

 product of the mucous membrane of the mouth, the ends of which are 

 frayed into fibres, and form a sieve-net for catching the little animals 

 which are engulfed with the sea- water. The mouth-cavity itself lias 

 become enormous, so that great quantities of water at a time can be 

 strained through the net of whalebone-plates. 



When I mention that peculiar changes have occurred also in the 

 internal organs, that the lungs have elongated longitudinall}' and thus 

 enable the animal more readily to lie in the water in a horizontal 

 position, that peculiar arrangements exist within the nostrils and the 

 larynx which enable the animal to breathe and swallow simultaneously, 

 and that the diaphragm lies almost horizontally because of the length 

 of the lung, I think I have said enough to indicate that not only does 

 almost everything about the whale diverge from the usual mammalian 

 type, but that all these deviations are adaptations to an aquatic life. If 



s? 



