262 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 



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 in- 

 directly connected with the conditions 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 sug- 

 g-estion cannot be accepted, for it is easy to show that all these 

 structural features depend upon adaptation to a peculiar mode 

 of feeding 1 . Functional 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 leng-th, 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 hug-e mouth which takes in a vast 

 quantity of water at a sing-le mouthful. The water then niters 

 away through the plates of whalebone, while the organisms which 

 form the whale's food remain stranded in the mouth. Is it neces- 

 sary to add that the internal organs so far as we understand the 

 details of their functions, and so far as their structure differs from 

 that of the corresponding organs in other Mammalia have also 

 been directly or indirectly modified by adaptation to an aquatic 

 life ? Thus all whales possess a very peculiar arrangement of 

 the nasal passages and larynx, enabling- them to breathe and 

 swallow at the same time : the lungs are of enormous length, and 

 thus cause the animal to assume a horizontal position in the water 

 without the exercise of muscular effort : in consequence of this 

 latter modification, the diaphragm extends in a nearly horizontal 

 direction : there are moreover certain arrangements in the vascular 

 system which enable the animal to remain under water for a con- 

 siderable time, and so on. 



And now, in reference to this special example, I will repeat 

 the question which I have asked before : ' If everything- that 

 is characteristic of a group of animals depends upon adaptation, 

 what remains to be explained by the operation of an internal 



