Page:Popular Science Monthly Volume 77.djvu/347

Rh I can not in this place analyze the peculiarities which give the austral fauna of these "Falklandia" strands their special impress but I may specially cite the trilobites which are astonishingly developed. I presume any competent student of northern faunas, being shown a series of these without knowledge of their origin, would pronounce them of early Devonian age and yet they are neither northern species nor, in any large degree, northern genera. While they bear the impress of boreal genera and resort to morphologic equivalencies thereto in fugitive epidermal structures which so richly characterize the boreal trilobites at this time, they are on the whole constructed on a series of modified types which hold their fundamental expression while developing minor details with the chronology normal to their succession at the north. The Phacopes are seldom true Phacopes, the Dalmanites seldom true Dalmanites, yet the same structural decorations and extravagances we are familiar with at the north, are distributed freely through the group. This is all equally true, in qualifying terms, of the other groups of this fauna, save for the fact that in these we can hardly venture to insist so entirely on generic distinctions south and north. The species differences declare themselves on every hand and taken as a whole the fauna presents fairly conclusive evidence of having derived its distinctiveness through its isolation from the boreal fauna from which it ancestrally took origin. Yet while it has developed this character it has also proceeded to maintain a faunal composition which declares its age, and a morphological stamp which shows that it developed all its parts in the proper time and place in the series.

In predicating geographic isolation as the prime factor in this regional development of the Devonian fauna, its efficiency should not be made to seem qualified by an illustration which is striking by virtue of its contrast with the already well known. There are evidences in plenty that geographic isolation has played a similar rôle with even more diverse effect in the development of the boreal faunas of the same geologic stage. The north Atlantic land bridge was continuous at this time, as evidenced not alone by the presence of the Coblentzian fauna in the Atlantic coast rocks but by an array of additional facts; and it seems very probable that the primary movement of these northern faunas was from the same African dispersion area as that of the south.