80 ZOOLOGY 



horns are likewise the dominating features of development. The 

 teeth, with the cunning alternations of harder and softer material 

 on their elaborate grinding surfaces, are indispensable imple- 

 ments in grinding into impalpable powder the hard and gritty 

 food on which the Ungulata live ; the hoofs are arrangements to 

 permit of long-continued running, and the horns are the final 

 defence of the animal against attack. The perfection of these 

 three sets of organs varies from group to group, and hence they 

 are made the basis of classification. This list could be ex- 

 tended indefinitely. In bivalve mollusca the gill, clothed 

 with ciliated epithelium which wafts the food-bearing current 

 of water to the mouth, and the prolongation of the mantle 

 termed the siphon (see Fig. 20), which enables the animal, 

 though buried in mud, to maintain connection with the vital 

 air-containing sea water above, are the dominating organs, and 

 classification is based on them. Among insects the modified 

 limbs round the mouth termed "jaws," or gnathites by the 

 aid of which they obtain and masticate their food, and the 

 wings which bear them from place to place are the basis 

 of classification. 



If the question be put, how can any but the most perfected 

 organisations survive until the present day] the answer is 

 twofold. First, competition and natural selection are not 

 equally severe in all areas. In small secluded areas less 

 perfect forms of life may survive for millions of years after their- 

 congeners have been eliminated everywhere else on the globe. 

 Thus in New Zealand the Tuatara, a primitive form of reptile, 

 still survives : its nearest allies swarmed all over Europe, Asia, 

 and Africa in the Triassic period. Again, all perfection in an 

 organ implies specialisation in function ; and the power oi 

 carrying out one function in perfection implies more or less the 

 giving up of the power of using the perfected organ for any 

 other purpose. Less perfection in structure may be compensated, 

 for by greater power of adaptability to various uses as the life 

 of the animal may demand. 



The paw of the dog is a much less perfect instrument than' 

 the paw of the cat for striking prey, but the paw of the dog* 

 can be used for long-continued running, whereas the cat is 8 

 only capable of a few short dashes at a time. The wild coii-r 

 gener of the dog, the wolf, can run down and tire out his pre$v 



