. BUFFALO SOCIETY OF NATURAL SCIENCES 200 



which came under the same environmental conditions might, and ex- 

 perience shows that they would develop along parallel lines, appear- 

 ing in later geological times as similar or even what might be called 

 identical species. In the course of centuries emigrants from an earlier 

 home centre of distribution would pass from the headwaters of one 

 stream to those of another, and soon these forms which had been pass- 

 ing through their individual modifications under one set of environ- 

 mental factors would migrate down the rivers and mingle with those 

 forms which had in an earlier period sought the lower reaches of the 

 rivers where a different complex of environmental factors obtained, 

 and there the old immigrants and the new, would come to live in the 

 same waters. A single family, in this way, would give rise to a cer- 

 tain number of primitive genera, some of which would migrate far from 

 the original centre of distribution. The descendants of these early im- 

 migrants might, after a long time and after having suffered profound 

 morphological changes, return to mingle with the descendants of the 

 provincial forms which had never left the ancestral region. Now let 

 us think of such inter-changes going on across the Nearctic continent 

 all through the Tertiary until at the close Europe was separated from 

 North America by an advance of the sea. At once we have two sepa- 

 rate continents and two river faunas. Were one to try to account 

 for the distribution of the fluviatile forms now living in the rivers by 

 a study of the present geography, one would be in despair to account 

 for the similarity or seeming identity of many species on opposite sides 

 of the dividing waters. Evidently the only mode of attack is by the 

 study of successively earlier and earlier fossil faunas and by the slow 

 reconstruction of the palaeogeography for each of those periods. 

 One need not search far to find the application of these hypothetical 

 statements to the eurypterids. If they were river-living organisms 

 then it is clearly impossible to explain their distribution in any par- 

 ticular period without considering their distribution in each immedi- 

 ately preceding period. No one has ever done this because each 

 writer tried to account for eurypterid occurrences on a hypothesis of 

 marine distribution. 



The results of migration are very different for marine organisms, 

 because of the fundamental difference between the continuity of the 

 seas and the discontinuity of the lands. Marine faunas, especially 

 the vagrant benthos of the littoral zone and the pelagic ones, tend to 

 be widespread, for they have greater freedom in the size of life dis- 

 tricts available, and in the lesser competition, as compared with the 



