ENVIRONMENTAL CLASSIFICATION 53 



aquatic animals, only cephalopods, crabs, and fishes are in any degree 

 comparable to land animals along this line, and social life, in the 

 strict sense, such as appears among insects, birds, and mammals, is 

 less perfectly developed among the primarily aquatic forms. Thus, 

 although the sea is its original home, animal life reaches its highest 

 development on land. 



In contrast with the great uniformity of habitat conditions in the 

 water, especially in the sea, there is a much greater diversity and 

 variability in the conditions to which terrestrial animals are subject. 

 In the sea the temperature variation has an amplitude of only 26°, 

 and only 20° at any one place. On land the temperature may go far 

 below zero and may rise to more than 60°C. The differences in 

 humidity are, of course, excluded in the aquatic habitat. The great 

 differences in the nature of the soil deeply affect animal life. The in- 

 fluence of topographic relief, on account of its climatic results, is more 

 impressive; and finally, the separation of land areas by water stands 

 in sharp contrast with the much greater continuity of the oceans. All 

 these factors together result in more diversity and complexity of adap- 

 tations and of directions of evolution, and favor to a high degree the 

 transformation of forms both in space and time. This explains the 

 much greater number of species of terrestrial as compared with aquatic 

 animals remarked upon above. It also explains the fact that genera of 

 aquatic animals extend so much farther back in geologic time, which is 

 a correlative of the more rapid evolution among land animals and the 

 shorter duration of the life of a species among them. 



The oldest genera of animals now living are aquatic forms — wit- 

 ness the brachiopod Lingula (Cambrian to Recent) and the gastropod 

 Pleurotomaria, which originated in the Silurian. The origins of our 

 present-day land snails date from the Eocene, those of the fresh-water 

 mollusks from far back in the Cretaceous. 39 Of the arthropods, many 

 genera of Crustacea can be traced back to the Mesozoic. Among 

 modern cirripedes, Pollicipes appears in the Jurassic, Scalpellum in 

 the Cretaceous. Among decapods Callianassa also appears in the 

 Jurassic, and Palinurus, Nephrops, and probably Homarus are known 

 from the Cretaceous. 40 In contrast with these, the genera of insects 

 range only into the Tertiary, and the genera and for the most part 

 even the families of the Jurassic are extinct. 41 This is even more 

 notable among the vertebrates. The genera of fishes have changed 

 relatively little since the Cretaceous; Ceratodus known from the 

 Triassic is very close to the living Neoceratodus ; many modern genera 

 of selachians are known from the Jurassic and Cretaceous; teleosts 

 like Clupea and Beryx appear in the Upper Cretaceous, and many 



