1 64 S. W. WILLISTON 



really, for most purposes, genera express in vertebrate paleontology 

 about what species suggest among invertebrates and plants, that is 

 for correlative and evolutional purposes, at least. 



The evolution of vertebrate life, air-breathing vertebrate life, for 

 I shall not presume to speak of the fishes, during Carboniferous times 

 was quite as great as at any subsequent period. Indeed, I think I 

 am quite safe in saying that, so far as the chief problems in vertebrate 

 evolution are concerned, the life of the Carboniferous is- the most 

 important of all. From forms scarcely differing from fishes which 

 must have existed at the beginning, of which, alas, we yet have no 

 knowledge, we find evolved at the close forms foreshadowing the 

 chief groups of life of modern times. The predominant types of 

 the Pennsylvanian were what we usually call the branchiosaurs and 

 microsaurs, for the most part small or very small creatures, at least 

 as small as their nearest relatives of the present time, the salamanders. 

 We are quite justified in the belief that their habits in general were not 

 greatly unlike these descendants, rather sluggish creatures living 

 about or in the water, for the branchiosaurs at least passed through 

 larval stages. They were more or less protected by an external 

 bodily armor against their enemies, whether of their own or other 

 kinds, in all probability terminating their existence as distinctive types 

 long before the close of the Paleozoic. But among them there were 

 some classed with the heterogeneous group which we call microsaurs, 

 which had made a very distinct advance, both toward a higher exist- 

 ence and away from the water. It has been assumed on entirely 

 insufficient evidence that they too were all amphibians, having an 

 early larval existence in the water, but of this we have, for many of 

 them, little or no proof, and there is very little to differentiate the 

 most advanced of them in structure from the reptiles. Some lost the 

 dermal armor completely and became fleet of movement, as is evi- 

 denced by the structure of their limbs, limbs mimicking in form and 

 in structure so closely those of modern quick-running lizards as to be 

 practically indistinguishable. We may be assured that some of them 

 before the close of the Pennsylvanian were inhabitants of high-and- 

 dry land regions where fleetness of movement, rather than obscurity, 

 preserved them from their enemies, crawling reptiles in everything 

 save some insignificant technical details of their palates. Specializa- 



