154 THE DESCENT OP MAN. 



a few general remarks. Every evolutionist will admit that the 

 five great vertebrate classes, namely, mammals, birds, reptiles, 

 amphibians, and fishes, are descended from some one prototype; 

 for they have much in common, especially during their embryonic 

 state. As the class of fishes is the most lowly organized, and 

 appeared before the others, we may conclude that all the members 

 of the vertebrate kingdom are derived from some fish-like animal. 

 The belief that animals so distinct as a monkey, an elephant, a 

 humming-bird, a snake, a frog, and a fish, &c., could all have 

 sprung from the same parents, will appear monstrous to those 

 who have not attended to the recent progress of natural history. 

 For this belief implies the former existence of links binding 

 closely together all these forms, now so utterly unlike. 



Nevertheless, it is certain that groups of animals have existed, 

 or do now exist, which serve to connect several of the great 

 vertebrate classes more or less closely. We have seen that the 

 Ornithorhynchus graduates towards reptiles; and Prof. Huxley 

 has discovered, and is confirmed by Mr. Cope and others, that 

 the Dinosaurians are in many important characters intermediate 

 between certain reptiles and certain birds — the birds referred 

 to being the ostrich-tribe (itself evidently a widely-diffused 

 remnant of a larger group) and the Archeopteryx, that strange 

 Secondary bird, with a long lizard-like tail. Again, according to 

 Prof. Owen,-^ the Ichthyosaurians — great sea-lizards furnished 

 with paddles — present many affinities with fishes, or rather 

 according to Huxley, with amphibians; a class which, including 

 in its highest division frogs and toads, is plainly allied to the 

 Ganoid fishes. These latter fishes swarmed during the earlier 

 geological periods, and were constructed on what is called a 

 generalized type, that is, they presented diversified affinities with 

 other groups of organisms. The Lepidosiren is also so closely 

 allied to amphibians and fishes, that naturalists long disputed in 

 which of these two classes to rank It; it, and also some few 

 Ganoid fishes, have been preserved from utter extinction by 

 inhabiting rivers, which are harbors of refuge, and are related 

 to the great waters of the ocean in the same way that islands 

 are to continents. 



Lastly, one single member of the immense and diversified class 

 of fishes, namely, the lancelet or amphioxus, is so different from 

 all other fishes, that Hackel maintains that it ought to form a 

 distinct class in the vertebrate kingdom. This fish is remarkable 

 for its negative characters; it can hardly be said to possess a 

 brain, vertebral column, or heart, &c.; so that it was classed by 



by Hackel, although he differs on some points. He expresses, also, 

 his high estimate of the general tenor and spirit of the whole work. 

 22 'Palaeontology,' 1860, p. 199. 



