Variation and Natural Selection. 113 



several years various species of grasshoppers appeared in 

 great numbers in South-east Bussia, there came then one 

 year of sudden death for most of them. They were sitting 

 motionless on the grasses and dying. He gives similar 

 cases of butterflies for a while numerous, and then rare, 

 and states that a squirrel common near Sarepta suddenly 

 disappeared in the course of one summer, probably, he 

 adds, succumbing to some contagious disease. Such is the 

 nice balance of life, that the partial disappearance of a 

 single form may produce remarkable and little-expected 

 effects. Darwin amusingly showed how the clover crops 

 might be beneficially affected by the introduction of a 

 family of old maids into a parish. The clover is fertilized 

 by humble-bees, the bees are preyed upon by mice ; the 

 relations between cats and mice, and between old maids 

 and cats, are well known and familiar : more old maids, 

 more cats ; more cats, less mice ; less mice, more humble- 

 bees ; more humble-bees, better fertilization. A little thing 

 may modify the balance of life, and increase or diminish 

 the struggle for existence, and the rigour of the process of 

 elimination. 



But when we take a more extended view of the matter, 

 and include secular changes of climate, the possible range 

 of variation in the struggle for existence is seen to be 

 enormously increased. It is well known to those who have 

 followed the progress of geology, that in early Kainozoic 

 times a mild climate extended to within the Arctic circle, 

 while during the glacial epoch much of the north tempe- 

 rate zone was fast locked in ice, and the climate of the 

 northern hemisphere was profoundly modified. The animals 

 in the north temperate zone were driven southwards.* 

 Not only was there much elimination from the severe 

 climatic conditions, but the migrants were driven south- 

 wards into areas already well stocked with life, and the 



* We may here note, in passing, the fact that the changes of life-forms in a 

 succession of beds points jn nine cases out of ten rather to substitution through 

 migration than to transmutation. Still, there are notable cases of trans- 

 mutation, as in the fresh-water Planorbes of Steinhem, in Wittenberg (described, 

 after Hilgendorf, by 0. Schmidt, " The Doctrine of Descent," p. 96). 



I 



