DEVELOPMENT OF VERTEBRATE LIFE IN PERMO-CARBONIFEROUS 271 



From the above it is evident that Matthew considers that peripheral 

 species of an expanding fauna are the most primitive and that species near 

 the point of origin show the greatest modification. This might be true under 

 certain conditions, but only if the migrant species followed a migrating 

 environment which advanced as an expanding belt without change and if the 

 original location underwent progressive change. It is obvious that this is 

 not always the case; in the Permo-Carboniferous radiation the change of 

 environment starting from the eastern side of the continent spread out, not 

 as an expanding belt, with other and new conditions arising at the original 

 source, but rather as an expanding blanket from a source where conditions 

 remained constant. In this case, certainly, the migrant forms would follow 

 the edges of the expanding environment, but the point of origin would offer 

 no stimulus for further- change and the fauna would be similar in all parts, 

 except that at the place of origin where the fauna had lived longest in close 

 association there would be closer adaptations in the interrelations of the 

 organic environment, and except as the factors of evolution which are 

 independent of the environment made themselves felt. In such highly 

 specialized forms as Edaphosaurus, as widely separated as Bohemia and 

 New Mexico, it is certain that one place, at least, is far removed from the 

 place of origin. There is, of course, the possibility, less in the reptiles 

 perhaps than in the mammals, of recurrent migration or of the active mi- 

 gration of groups during really short intervals of time, within an environment 

 very similar over large areas ; this might cause certain forms to become very 

 widely spread and to confuse the interpretation of the fossil record. 



With the spread of Permo-Carboniferous conditions, the fauna of the 

 time extended its range over a large part of the United States at least, 

 if not over a large part of the continent. Whether the fauna occupied all 

 parts of its extreme range at any one time is uncertain, but it is certain that 

 in late Paleozoic, true Permo-Carboniferous time, it extended from Penn- 

 sylvania to New Mexico, and it is very probable that it existed in regions 

 where no remains have been discovered. The large proportion of exposed 

 land in the eastern and southern parts of the United States permitted little 

 preservation of the remains of the animals, if any were present. Beyond 

 New Mexico we have no record and in the northwest marine conditions 

 prevailed during Gschelian time and gave place to elevated land, with the 

 accompaniment of volcanic disturbances and earth-movements which prob- 

 ably prohibited the presence of the peculiar vertebrate life, though it per- 

 mitted the passage of the fern Gigantopteris . 



Permo-Carboniferous vertebrate land life came completely to an end 

 in North America and, so far as we can tell, suddenly. Inadequate 

 history makes it impossible to draw conclusions which approach to finality, 

 but the chances for the discovery of far-reaching evidence are seemingly 

 so small that it is well to summarize what can be said from that now 

 available. 



