The- Theory of Evolution 55 



groups could have arisen, and that ornithorynchus cannot 

 be placed as an intermediate form, a link between saurian s 

 and mammals, as the followers of the transmutation theory 

 maintain. He shows, giving citations, that anatomists them- 

 selves are by no means in accord as to the exact position of 

 ornithorhynchus in relation to the higher forms. 



In reply to this criticism, the same answer made above for 

 archaeopteryx may be repeated here, namely, that because cer- 

 tain optimists have declared the monotremes to be connecting 

 forms, it does not follow that the descent theory is untrue, and 

 not even that these forms do not give support to the theory, 

 if in a less direct way. I doubt if any living zoologist regards 

 either ornithorhynchus or echidna as the ancestral form from 

 which the mammals have arisen. But on the other hand it 

 may be well not to forget that these two forms possess many 

 characters intermediate between those of mammals and rep- 

 tiles, and it is from a group having such intermediate characters 

 that we should expect the mammals to have arisen. These 

 forms show, if they show nothing else, that it is possible for a 

 species to combine some of the characters of the reptiles with 

 those of the mammals ; and the transmutation theory does no 

 more than postulate the existence at one time of such a group, 

 the different species of which may have differed in a number 

 of points from the two existing genera of monotremes. 



The origin of lung-bearing vertebrates from fishlike ances- 

 tors, in which the swim-bladder has been changed into lungs, 

 has been pointed to by the advocates of the transmutation 

 theory as receiving confirmation in the existence of animals 

 like those in the group of dipnoan fishes. In these animals 

 both gills and a swim-bladder, that can be used as a lung, are 

 present; and through some such intermediate forms it is 

 generally supposed that the lung-bearing animals have arisen. 

 Fleischmann argues, however, that, on account of certain 

 trivial differences in the position of the duct of the swim- 

 bladder in living species, the supposed comparison is not to 



