180 THE DESCENT OF MAN. 



remarks. Every evolutionist will admit that the five great 

 vertebrate classes, namely, mammals, birds, reptiles, amphib- 

 ians and fishes, are descended from some one prototype; for 

 they have much in common, especially during their embry- 

 onic state. As the class of fishes is the most lowly organ- 

 ized, 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, etc., 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 toward reptiles; 

 and Prof. Huxley has discovered, and is confirmed by Mr. 

 Cope and others, that the Dinosaurians are in many impor- 

 tant 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, 

 Avith 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 



" Nattirliclie ScliSpfungsgeschiclite," 1868. Prof. Huxley, in review- 

 ing this latter work (" The Academy," 1869, p. 42) says, that he con- 

 siders the phylum or lines of descent of the vertebrata to be admir- 

 ably discussed by Hackel, although he differs on some points. He 

 expresses, also, his high estimate of the general tenor and spirit of 

 the whole work. 

 *" Palaeontology, " 1860, p. 199. 



