CLIMATIC CHANGES 327 



of them have some special character which enables them to be 

 easily dispersed by external agencies. The fresh water Polyzoa 

 and sponges produce minute buds (statoblasts or gemmules) 

 enclosed in hard protective envelopes (Figs. 165, 166), which are 

 likely to be carried about in the mud on the feet of wading birds 

 and mammals. The embryo of Hydra secretes its own protective 

 envelope (Fig. 59, D G) within which it passes through a period 

 of rest embedded in the mud ; while many of the Protista (e.g. 

 Hsematococcus) are capable of being dried up at some period or 

 other of their life-history and carried about by the wind in the 

 form of dust. Thus a sample of mud, taken from a pond and 

 dried up, may, after an interval of many months, if again placed 

 in water, give rise to an abundant fauna, amongst which even 

 Buch highly organized forms as Crustacea (e.g. Apus and 

 Branchipus, which lay specially protected eggs) will frequently 

 appear. 



We must remember that the present distribution of animals 

 and plants is the outcome not only of the existing physical 

 conditions of the earth's surface but also of conditions which 

 obtained in past geological periods. From time to time these 

 conditions undergo great changes, which may concern not only 

 the climate of particular regions or of the entire world, but also 

 the relative distribution of land and sea. 



The earth has been subject, at various periods of its history, 

 to climatic changes of two chief kinds, (1) cold or even glacial 

 epochs in temperate regions, and (2) mild or warm epochs in 

 arctic or antarctic regions. Probably alternations of these two 

 extremes have been not infrequent, but the case of which we 

 have most complete knowledge occurred in the Pleistocene period 

 and is usually known as the " Glacial Epoch " par excellence. 



There is clear evidence that during a portion of the Pleistocene 

 period a very large part of the northern hemisphere, which now 

 enjoys a temperate climate, was covered with perpetual ice and 

 snow and reduced to a condition resembling that of Greenland 

 at the present time. Scandinavia and the whole of Northern 

 Europe were buried beneath the ice-sheet, and the same is truo 

 of the northern part of North America. 



The glacial epoch in the north must have driven the greater 

 number of the northern plants and animals southwards, causing 

 a keen struggle for existence in which many species were 

 exterminated. Its influence was possibly intensified by the 



