432 NATURAL SCIENCE. ^^^^^^_ 



cation of these views ; but it should not be forgotten that the northern 

 shores of the Mediterranean have been well explored geologically, 

 and that Italy and Sicily especially have long been classic ground for 

 the student of Pleistocene geology. 



To indicate more clearly the probable condition of Europe during 

 the greatest intensity of the cold, I have ventured to construct an 

 isothermal map from the data now available. Of course, the 

 isotherms are mere rude approximations, to be corrected and 

 improved as materials accumulate ; but for the purpose of the student 

 of natural history, and especially of geographical distribution, I 

 think that the map may be useful. It will show at least that 

 extensive regions must have been quite uninhabitable for their 

 present fauna and flora. In other regions the crowding of the 

 isotherms would probably bring into competition species that seldom 

 meet under present conditions ; and this crowding may help to 

 account for some of the anomalies in the occurrence of the Pleistocene 

 mammalia. A great deal of the supposed discordance in the evidence 

 results, however, from careless observation, and from the deposition 

 of already fossilised bones among others of later date. My own 

 observations have shown few genuine cases of discordant evidence 

 as to climate among the mammals, and none at all in other groups. 



An objection, apparently fatal, will occur to any naturalist who 

 attempts to verify the conclusions above arrived at. He will remark 

 that surely in all the thousands of years that have passed since the 

 origin of existing species, many forms must have become acclimatised, 

 and if the physical change were sufficiently slow the animals and 

 plants would accommodate themselves to the new conditions. Had 

 this question been put a few years since I should have felt that the 

 objection was perfectly valid ; but one of the most unexpected results 

 of the study of later Tertiary natural history has been to show that, 

 in a wild state, species seldom become acclimatised. They are 

 exterminated, or die out locally, or, perhaps, in a few cases, change 

 into other so-called species, with each climatic change ; but every 

 recurrence of cold or warm conditions causes the reappearance of 

 the appropriate fauna and flora. 



Only one phase of the Glacial Epoch, the lowest temperature 

 reached, has been dealt with in these notes. It still remains to 

 speak of interglacial mild episodes, of non-glaciated areas surrounded 

 by ice (" Nunataks," as they are called in Greenland), of arid 

 episodes, and of the extremely hypothetical " Pluvial Period." 

 These subjects can, perhaps, be better treated of in a separate 

 article, but it may be advisable to give a word of caution to 

 naturalists who are inclined to treat the severity of the cold and the 

 accumulation of snow as the only climatic factors to be taken into 

 account. Recent observations have shown more and more clearly 

 that throughout Central Europe there was also a period of dry cold, 

 causing that region to resemble the arid wastes of Central Asia. 



