104 EVOLUTION, SOCIAL AND ORGANIC 



rels, for instance, when there is a scarcity of 

 cones in the larch forests, remove to the fir- 

 tree forests, and this change of food has cer- 

 tain well known physiological ef5fects on squir- 

 rels. If this change of habits does not last — 

 if next year the cones are again plentiful in 

 the dark larch wood — no new variety of 

 squirrels will evidently arise from this cause. 

 But if part of the wide area occupied by the 

 squirrels begins to have its physical charac- 

 ters altered — in consequence of, let us say, 

 a milder climate or desiccation, (drying up) 

 which both bring about an increase of the pine 

 forests in proportion to the larch woods — 

 and if some other conditions occur to induce 

 squirrels to dwell on the outskirts of the de- 

 siccating region — we shall then have a new, 

 1. e., an incipient new species of squirrels. A 

 larger proportion of squirrels of the new, bet- 

 ter-adapted variety would survive each year, 

 and the intermediate links would die in the 

 course of time, without having been starved 

 out by Malthusian competitors." 



Again: "If we take the horses and cattle 

 which are grazing all the winter through in 

 the Steppes of Transbaikalia, we find them 

 very lean and exhausted at the end of the 

 winter. But they grow exhausted not because 

 there is not enough food for all of them — 



