Chap. VI.] AFFINITIES AND GENEALOGY 195 



The Monotremata are eminently interesting, as in several 

 important points of structure they lead toward the class 

 of reptiles 



In attempting to trace the genealogy of the Mam- 

 malia, and therefore of man, lower down in the series, we 

 become involved in greater and greater obscurity. He 

 who wishes to see what ingenuity and knowledge can 

 effect, may consult Prof. Hackel's works. 19 I will con- 

 tent myself with a few general remarks. Every evolu- 

 tionist will admit that the five great vertebrate classes, 

 namely, mammals, birds, reptiles, amphibians, and fishes, 

 are all 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 ap- 

 peared before the others, we may conclude that all the 

 members of the 'vertebrate kingdom are derived from some 

 fish-like animal, less highly organized than any as yet 

 found in the lowest known formations. The belief that 

 animals so distinct as a monkey or elephant and a hum- 

 ming-bird, a snake, frog, and 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 ex- 

 istence of links closely binding 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 more or 

 less closely the several great vertebrate classes. We have 



19 Elaborate tables are given in his ' Generelle Morphologie ' (B. it 

 s. cliii. and s. 425) ; and with more especial reference to man in his 

 'Natiirliche Schbpfungsgeschichte,' 1868. Prof. Huxley, in reviewing 

 this latter work ('The Academy,' 1869, p. 42) says that he considers 

 the phylum or lines of descent of the Vertebrata to be admirably dis- 

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

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

 whole work. 



