DEVELOPMENT OF VERTEBRATE LIFE IN PERMO-CARBONIFEROUS 265 



Environmental monotony would result in the persistence of older and 

 simpler types because the variants, possibly being constantly produced, would 

 not have a chance to develop. 



Another result of the long association of the unchanging or only slowly 

 changing members of the fauna would be a very close adjustment of the 

 interrelationships of the various elements of the fauna, and of the fauna 

 with the flora and the inorganic environment. In a region of large possi- 

 bilities of varied habitat such persistence, amounting to static conditions, 

 might result in the production of highly specialized types showing excessive 

 morphological characters, but in a region of limited possibilities of habitat 

 the morphological expression of close adjustment would be less obvious. 

 Close adaptation of the interrelationships is in itself an evidence of the 

 long association of comparatively fixed groups, and when this finds expression 

 in the skeletal structure it can easily be read in the fossils, but if it is expressed 

 in physiological characters or in habits not revealed in the structure, as pecu- 

 liarities of feeding, etc., or in other things which may seem to the observer 

 minute and unimportant but are in reality vital, the record is not decipherable. 



Again, as has been shown by Beecher, certain morphological peculiarities 

 are produced only in the senility of a group and are characteristic of it; 

 the upper Paleozoic air-breathing vertebrates were still in the best stage of 

 their development. Though amphibians appeared in the Devonian and 

 footprints record their occurrence through the Mississippian and early 

 Pennsylvanian, their progress seems to have been very slow, a condition 

 which is analogous to that of the mammals in the Mesozoic. 



We have as yet no knowledge of the place of origin or the direction of 

 migration of the Permo-Carboniferous fauna, and speculation upon changes 

 induced by migration into new regions must be based on very meager 

 evidence. It is altogether possible that in their slow development the 

 primitive amphibia migrated only into extensions of their original environ- 

 ment and so experienced little change in their surroundings and received 

 no stimulus to evolution. The almost purely aquatic character of the 

 primitive amphibians and the intolerance to salt water, so characteristic of 

 the living, and presumably of the fossil, forms would tend to restrict their 

 movements most decidedly. Only when the as yet undetermined influence 

 which compelled a change appeared or gathered sufficient force to cause a 

 vigorous development would they burst through the physiological barriers 

 and begin their radiation. 



The upper Pennsylvanian air-breathing vertebrate fauna was young, 

 very numerous in individuals, possibly in kinds, but restricted in its further 

 development by the monotony of the environment. It was, however, 

 accumulating force towards a great radiation to be expressed as soon as the 

 limitations were removed, even in a partial degree. 



