48 THE BOOK OF NATURE STUDY 



There remain yet one or two other peculiarities of the mole 

 which demand special notice. In the first place, the eyes have 

 become reduced to the merest vestiges, so that they must be 

 carefully sought for among the fur, when they will be found as 

 two tiny black points. It is obvious that large eyes would not 

 only be useless to a creature living underground, but they would 

 be a source of danger, since the soil could not be kept from their 

 surfaces if they were opened. Secondly, there are no external 

 ears. These also would be constantly exposed to injury, and 

 would further impede progress. Finally, the fur differs from 

 that of other animals in that it can be brushed, and made to lie, 

 either backwards or forwards, with equal ease, the separate hairs 

 of the coat being thicker in the middle than at the base or tip. 

 Obviously, this is an advantage to an animal which, when moving 

 quickly forwards, may have to retreat speedily backwards in a 

 passage too small to make turning round possible. 



How completely the shapes of animals are determined by 

 their mode of life is further illustrated by the whales and por- 

 poises. 



Our forefathers, like many people to-day, regarded these 

 creatures as fish. Yet they are indubitable mammals, as their 

 anatomy incontestably proves. Thus the skeleton, nervous 

 system, heart and blood vessels, all differ absolutely from those 

 of the fishes, and agree with all other mammals. Like all the 

 mammalia, their young are nourished on milk ; and, like all the 

 mammalia, they breathe by lungs, and not by gills, as in fishes. 

 But they are mammals which have become slowly transformed 

 into creatures of fish-like form by reason of their exclusively 

 aquatic habits. Though to-day only minute vestiges of the 

 hind-limbs are found embedded within the muscles of the body 

 wall, there is no doubt but that the remote ancestors of these 

 animals possessed four limbs, and lived upon dry land. The 

 evidence for this would be too technical for the purpose of 

 this book. Suffice it here to outline the chief peculiarities of 

 whales and porpoises as we know them to-day. 



In the first place, they have lost all trace of hairs on the body, 

 the skin now being black in colour, and of wonderful smoothness. 

 The fore-limbs have become modified to form swimming organs 



