8 THE STUDY OF ANIMAL LIFE CHAP. 



ascent from sea to shore, from shore to dry land, and 

 eventually into the air, and the fact that the haunts and 

 homes of animals are not less varied than the pitch of 

 their life. In addition to minor haunts such as brackish 

 water, burrows, caves, and the interior of other animals, 

 six main haunts of life may be distinguished the sur- 

 face of the sea (pelagic), the depths of the sea (abyssal), 

 the shore (littoral), the fresh waters, the dry land, and 

 the air. 



3. Wealth of Form. As our observations accumu- 

 late, the desire for order asserts itself, and, like our 

 forefathers, we cannot help classifying, allowing similar 

 impressions to draw together into groups, such as birds 

 and beasts, fishes and worms. At first sight the types 

 of architecture seem confusingly numerous, but gradually 

 certain great samenesses are discerned. Thus we dis- 

 tinguish as higher animals those which have a supporting 

 rod along the back, and a nerve cord lying above this ; 

 while the lower animals have no such supporting rod, 

 and have their nerve-cord (when present) on the under, 

 not on the upper side of the body. The higher or back- 

 boned series has its double climax in the Birds and the 

 furred Mammals. Indissolubly linked to the Birds are 

 the Reptiles, lizards and snakes, tortoises and croco- 

 diles the survivors of a great series of ancient forms, 

 from among which Birds, and perhaps Mammals also, 

 long ago arose. Simpler in many ways, as in bones and 

 brains, are Amphibians and Fishes in close structural 

 Dalliance, with the strange double-breathing, gill- and 

 lung-possessing mud-fishes as links between them. Far 

 more old-fashioned than Fishes, though popularly in- 

 cluded along with them, are the Round-mouths the half- 

 parasitic hag-fish, and the palatable lampreys, with 

 quaint young sometimes called " nine-eyes." Near the base 

 of this series are the lancelets, small, almost translucent 

 animals living in the sea-sand. Just at the threshold of 

 the higher school of life, the sea-squirts or Tunicates 

 have for the most part stumbled ; for though the active 

 young forms -like minute tadpoles are indisputable Ver- 

 tebrates, almost all the adults fall from this estate, and 



