292 AIR-BREATHERS OF THE COAL PERIOD. 



Mollusks. This at once excludes the supposition that they can 

 all hare been derived from each other, -within the limits of the 

 coal period. No transmutationist can have the hardihood to as- 

 sert the convertibility, by any direct method, of a snail into a 

 millipede, or of a millipede into a reptile. The plan of structure 

 in these creatures is not only different but contrasted in its most es- 

 sential 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 stageSj on the principle 

 of natural selection, whereby these results might be effected ; but 

 the hypothesis would be destitute of any support 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 Limned 1 } It will not solve this difficulty to say that the 

 intermediate forms became extinct and so are lost. On the con- 

 trary 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. Here, if anywhere, in what appears 

 to be the first introduction of air-breathing invertebrates, we should 

 be able to find the evidences of transition from the gills of the 

 prosobranchiate and the crustacean to the air sac of the pulmonate 

 and the tracheae of the millipede. It is also to be observed that 

 many other structural changes are involved, the aggregate of 

 which makes a pulmonate or a millipede different in every par- 

 ticular from its nearest allies among gill-bearing gasteropods or 

 crustaceans. 



It may be said however that the links of connection between 

 the coal reptiles and the fishes are better established. All the 

 known coal reptiles have leanings to the fishes in certain charac- 

 ters ; and in some, as in Archego$auru$, these are very close. Still 



