308 COSMIC PI1ILOSOPIIY. [it. il 



consider the species instead of the individual, we shall find 

 that still longer astronomic rhythms, often complicated by 

 geologic rhythms, cause periodic changes in the total mani- 

 festations of life upon the earth's surface. Recurring epochs 

 of high eccentricity of the earth's orbit have so altered the 

 distribution of solar radiance as to cause violent climatic 

 vicissitudes. Large portions of the earth have been covered 

 by glaciers, and there have been ensuing migrations of plants 

 and animals, attended by the extinction of many forms, and 

 by specific variations among the survivors. Other rhythms 

 in the distribution of life have been caused by alternations 

 in the elevation and subsidence of continents and islands. 

 And all the foregoing causes, taken altogether, have been 

 endlessly complicated by rhythmic changes in the relations 

 of various groups of organisms to one another. The com- 

 plexity of such relations is strikingly illustrated in an 

 instance given by Mr. Darwin. The fertilization of hearts- 

 ease and red clover is impossible without the agency of 

 humble-bees in carrying the pollen from one flower to 

 another. Other bees do not visit these flowers, as their 

 probosces are not long enough to reach the nectar; while 

 moths, which have sufficiently long probosces, are not heavy 

 enough to bend down the petals in such a way that the 

 anthers above may shed pollen upon their backs. Hence 

 the partial or total destruction of humble-bees must involve 

 the decrease or extinction of heartsease and red clover. But 

 observation shows that the mortal foes of humble-bees are 

 field-mice, who destroy their combs and nests. It is esti- 

 mated that in England more than two-thirds of each 

 generation of humble-bees are destroyed by mice. Hence 

 it follows that the cat is a friend and protector of the 

 humble-bee ; and that any sensible variation in the number 

 of cats in a given district must indirectly cause a variation 

 in the numbers of heartsease and red clover which grow in 

 the neighbourhood. It is only needful to add that in such 



