48 THE THEORY OF NATURAL SELECTION 



The external openings of the nostrils are placed quite 

 on the upper surface of the head, so that the animal 

 can breathe whilst almost completely submerged ; and 

 the larynx is so modified that the function of swallowing 

 does not interfere with that of breathing. Perhaps the 

 most remarkable feature of all is the enormous develop- 

 ment of the head, and especially of the mouth. The 

 huge jaws, in combination with the extraordinary plates 

 of whalebone which fringe the edges of the mouth and 

 act as a sieve, enable the animal to get its nutriment 

 from the minute free-swimming creatures with which 

 the surface waters of the ocean abound. Associated 

 with this special method of feeding is the fact that 

 teeth are only to be recognised in the embryo, and 

 afterwards entirely disappear. 



The whales differ in all these points from any other 

 mammals, and failing almost any of these differences, 

 they would not be able to live in the special conditions 

 in which they find themselves. It must therefore be 

 admitted that we have here a case of very close adapta- 

 tion of an animal to its natural surroundings, and one 

 which extends to almost every detail of its structure. 

 Darwin himself, moreover, has been at special pains to 

 show how some of the most remarkable of these 

 structural adaptations may possibly have arisen 

 through natural selection. 



One of the most remarkable cases of mutual adapta- 

 tion, in which an animal and a plant are associated 

 together, is shown by the method of fertilization 

 observed to take place in the flowers of the Yucca 

 plant of the Southern United States. The act of 



