No. 410.] THE EUROPEAN FAUNA. IOI 
argument, rest upon the fundamentally wrong idea that whole 
assemblages of species — or single species for that matter — 
can accommodate themselves to so different conditions without 
changing their specific identity. Surely the examples which 
Dr. Scharff mentions, while referring to the difficulty of keep- 
ing southern animals and plants alive in northern climates, 
and, vice versa, the greater ease with which northern animals 
survive in temperate countries, refer to individuals only, though 
he speaks of species. It is a pretty well established fact by 
this time that the distribution of an animal or plant (species) 
is limited within a certain life zone beyond which it cannot 
proceed with impunity, and that this life zone is bounded by 
certain isotherms of the propagating season, boundaries which 
may differ with each species but which are fairly inflexible 
within the species. This Zzzv, which Dr. Merriam has so suc- 
cessfully defined and elaborated, renders it certain beyond a 
doubt that the presence in central Europe of a breeding and 
propagating assemblage of animals and plants practically identi- 
cal with that of the Arctic and sub-Arctic regions of to-day 
indicates a corresponding climate during the propagating sea- 
son; in other words, the life zone of these Arctic and sub- 
Arctic species was at some period during the glacial epoch 
located in central Europe, plants, animals, temperature, and all. 
Another reason for Dr. Scharff's adoption of the theory of 
a mild glacial climate is the alleged marine origin of the 
boulder clay. "Various deposits of marine invertebrate fossils 
in stratified beds are cited as proof that the boulder clay is not 
of the nature of a ground moraine. While this question prob- 
ably is one in which the geologists are more directly concerned, 
I may say that, in my humble opinion, the most reasonable 
. explanation is that the boulder clay is of a dual origin, — that 
part of it is deposited on land by the ice sheet, while other 
Portions were formed at the bottom of the sea, dropped by 
floating ice and bergs. There can be but little doubt that the 
€normous weight of the Scandinavian ice cap depressed the 
land to the south of it, so as to bring it under the level of 
the sea. There was probably always a more or less extensive 
Sea skirting it to the south and east, preceding the glaciers as 
