198 SUMMARY OF CURRENT RESEARCHES DELATING TO 



origin. Hence there can be slight stratigraphic record of the distributory 

 movements of faunas and floras in the Palaeozoic, though even then 

 polar climates were probably the most important of evolutionary factors. 

 From the origin of life down to the Mesozoic the north and south polar 

 areas may have played a nearly equal part in creating a certain south- 

 ward and northward stress, together with a sort of breaking up of species 

 in the tropics. But from the Mesozoic to the Glacial period, evidence 

 points to the polar origin, and continuous outward dispersion from the 

 north polar area of most of the great plant and vertebrate groups. The 

 similarity in successive unrelated and diverse faunas synchronously 

 appearing on both sides of the Atlantic cannot be accounted for through- 

 out long periods of time on the basis of lateral interchange. The record 

 of the post-Palaeozoic flora is in all essentials the complement of the 

 vertebrate record, and far more complete. Moreover, the outward move- 

 ment, especially of Conifers and Dicotyledons, from the Arctic area for 

 long periods of time, has frequently been recognised. Some traces of 

 this movement are still evident in the present strikingly homogenous 

 circumboreal flora, although its main development was obscured and 

 partially checked by the appearance of glacial conditions. It seems con- 

 clusive that all the factors of climate — and, therefore, the main alterna- 

 tive potentialities producing organic evolution — have been in the highest 

 degree variant in the polar areas. This being true, the grouping of the 

 continents about the north polar area would render it probable, were 

 there not abundant direct evidence pointing to the fact, that the northern 

 circumpolar area has probably been, ever since the older Palaeozoic at 

 least, the main evolutionary centre from which animal and plant life 

 has radiated. This view is supported by overwhelming proof that it is 

 from the Arctic area that the greatest waves of change have swept out 

 to lessen and disintegrate in the more static conditions of the tropics. 



Racial Variation.* — Witmer Stone, who has for a number of years 

 studied racial variation amongst terrestrial vertebrates, and its relation 

 to environment and climatic conditions, has recently investigated the 

 genus Viola in the neighbourhood of Philadelphia on the same lines. 

 His studies, while throwing little light on the relation of variation to 

 conditions of environment, have enabled him to give a fairly complete 

 account of the variations shown by the locali violets, which may be a help 

 to those interested in the genus. The author emphasises his view of 

 the advantage of the trinominal for denoting races. Among terrestrial 

 vertebrates racial variation corresponds closely to geographic environ- 

 ment, and in many groups it is easy to recognise the effect of the 

 environment on several different life-areas in producing recognisably 

 distinct races from the same type. As is well known, trees and shrubs, 

 as well as other plants in a less degree, conform with more or less exact- 

 ness to the same general laws of geographic distribution that pertain to 

 animals ; and the ranges of many species are limited by the life-zones 

 established originally from a study of birds and mammals. When, how- 

 ever, a genus is represented by different forms in several life-zones, they 

 are usually very distinct species, and not closely related variants which 



• Proe. Acad. Nat. Sci. Philadelph., 1903, pp. 65C-99 (9 pis.). 



