1054 LIFE : OUTLINES OF GENERAL BIOLOGY 



and instructive inquiry to associate temporal variations in the life- 

 curve with seasonal and with climatic periodicities. 



The Arctic tundra is marked by a long, dark winter of bitter cold 

 and a short nightless summer of intense illumination; therefore it 

 is reasonable to postulate a prolonged process of elimination as the 

 climate changed — an elimination of those types which did not vary 

 in the direction of quick-flowering and quick-fruiting, dying down 

 in winter, dispensing with all but a little water, and accumulating 

 stores in underground parts. 



Among the features of the life-curve that may be tentatively 

 associated with climatic changes, the following may be suggested — 

 the length of the mammalian gestation and the time of giving birth ; 

 the periodicities of migrating birds and even peculiar cases like 

 that of the cuckoo, where the adults leave the north long before the 

 young ones; the tendency to shorten the juvenile period when the 

 conditions are particularly hard on youth; the interpolation of 

 periods of winter-sleep, rest, coma, lethargy, and even de-differentia- 

 tion. Every trajectory of life should be looked at in the light of the 

 evolution of climates. 



Often in the history of the earth a change towards great cold 

 has involved severe elimination. In humid periods there tends to 

 be abundant succulent fodder for browsing animals and extension 

 of shelter-giving forests. Diminution of moisture, if it does not go 

 too far, favours the increase of grasses and of grazing animals. 

 Aridity makes the forests shrink, and prompts the search for new 

 haunts. Here reference may be made to the work of Lull (1917), 

 who has expounded and illustrated the effect of environmental 

 changes on the evolutionary stream. The times of quickening, the 

 "expression-points" or "pulsations" of evolution, may often be 

 correlated with climatic changes, chiefly in temperature and 

 humidity, due sometimes to topographic, at others to general 

 atmospheric conditions. Behind these, again, lie larger factors still, 

 such as shrinkages of the earth's crust. See also Matthew (1915.) 



INTERACTION OF ENDEMIC AND INTRODUCED FAUNAS. 



— (i) The newcomers may destroy the indigenous or previously 

 naturalised forms. Thus the mongoose, introduced into Jamaica, 

 destroyed the indigenous "cane-rats" and the alien ship-rats. Or 

 again, the introduction and diffusion of the domestic cat in Britain 

 must have checked the increase of mice ; and so, on a larger scale, 

 have other carnivores done service in the colonies, like weasels in 

 New Zealand. 



(2) The introduced animals may become so numerous that they 

 make life dilhcult for their predecessors, though they do not actually 

 devour them. They may, for instance, seriously reduce the food 

 supply, but they may be prejudicial with varying degrees of direct- 



