CHAP, t The Wealth of Life 9 



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 distin- 

 guish 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 backboned 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 crocodiles — 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 alliance, 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 is the lancelet, a small, almost translucent 

 animal living in the sea-sand at considerable depths. 

 It may be regarded as a far-off prophecy of a fish. 

 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 have been acknowledged 

 for many years as reputable Vertebrates, almost all the 

 adults fall from this estate, and become so degenerate that 

 no zoologist ignorant of their life-history would recognise 

 their true position. Below this come certain claimants for 

 Vertebrate distinction, notably one Balatioglossus, a worm- 

 like animal, idolised by modern zoology as a connecting link 

 between the backboned and backboneless series, and 

 reminding us that exact boundar>^-lines are very rare in 

 nature. For our present purpose it is immaterial whether 

 this strange animal be a worm -like vertebrate or a 

 vertebrate-like worm. 



Across the line, among the backboneless animals, it 

 is more difficult to distinguish successive grades of higher 



