CHAPTER X. 

 THE OLDEST AIR-BREATHERS. 



ANIMAL life had its beginning in the waters, and to 

 this day the waters are the chief habitat of animals, 

 especially of the lower forms. If we divide the animal kingdom 

 into great leading types, the lowest of these groups, the 

 Protozoa, includes only aquatic forms; the next, that of the 

 coral animals and their allies, is also aquatic. So are all the 

 species of the Sea Urchins and Star Fishes. Of the remaining 

 groups, the Mollusks, the Crustaceans, and the Worms are 

 dominantly aquatic, only a small proportion being air-breathers. 

 It is only in the two remaining groups, including the Insects 

 and Spiders on the one hand, and the Vertebrate animals on 

 the other, that we have terrestrial species in large proportion. 



The same fact appears in geological time. The periods 

 represented by the older Palaeozoic rocks have been termed 

 ages of invertebrates, and they might also be termed ages 

 of aquatic animals. It is only gradually, and as it were with 

 difficulty, that animals living in the less congenial element of 

 air are introduced at first a few scorpions and insects, later, 

 land snails and amphibian reptiles, later still, the higher rep- 

 tiles and the birds, and last of all the higher mammalia. 



We need not wonder at this, for the conditions of life with 

 reference to support, locomotion, and vicissitudes of temper- 

 ature are more complex and difficult in air, and require more 

 complicated and perfect machinery for their maintenance. 

 Thus it was that probably half of the whole history of our 



