304 THE OLDEST AIR-BREATHERS 



quite distinct in their nature and affinities. This at once ex- 

 cludes the supposition that they can all have been derived from 

 each other within the limits of the coal period. No transmu- 

 tationist can have the hardihood to assert the convertibility, by 

 any direct method, of a snail into a millipede or an insect, or 

 of either into a reptile. The plan of structure in these crea- 

 tures is not only different, but contrasted in its most essential 

 features. It would be far more natural to suppose that these 

 animals sprang from aquatic species of their respective types. 

 We should then seek for the ancestors of the snail in aquatic 

 Gasteropods, for those of the millipede in worms or Crustaceans, 

 and for those of the reptiles in the fishes of the period. It 

 would be easy to build up an imaginary series of stages, on 

 the principle of natural selection, whereby these results might 

 be effected ; but the hypothesis would be destitute of any sup- 

 port from fact, and would be beset by more difficulties than it 

 removes. Why should the result of the transformation of 

 water snails breathing by gills be a Pupa ? Would it not much 

 more likely be an Auricula or a Limnea ? It will not solve 

 this difficulty to say that the intermediate forms became ex- 

 tinct, and so are lost. On the contrary, they exist to this day, 

 though they were not, in so far as we know, introduced so 

 early. But negative evidence must not be relied on; the 

 record is very imperfect, and such creatures may have existed, 

 though unknown to us. It may be answered that they could 

 not have existed in any considerable numbers, else some of 

 their shells would have appeared in the coal-formation beds, so 

 rich in crustaceans and bivalve mollusks. Further, the little 

 Pupa remained unchanged during a very long time, and shows 

 no tendency to resolve itself into anything higher, or to descend 

 to anything lower, while in the lowest bed in which it occurs 

 it is associated with a round snail of quite different type. 

 Here, if anywhere, in what appears to be the first introduction 

 of air-breathing invertebrates, we should be able to find the 



