390 S. W. WILLISTON 
found to occupy two remote provinces would furnish more positive 
evidence of contemporaneity and the possibilities of faunal migrations 
than would scores of others of lower types. But of species in verte- 
brate paleontology we can say little; the term with us is usually a 
far more vague and indefinite one than it is among students of living 
faunas, partly because much of the evidence which the neozodlogist 
has, the paleozodlogist has not, partly because the taxonomy of living 
creatures is still based too much upon superficial resemblances. And 
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 
