RHYTHM 



total manifestations of life upon the earth's sur- 

 face. 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, at- 

 tended 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 sub- 

 sidence 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 complexity of such relations 

 is strikingly illustrated in an instance given by 

 Mr. Darwin. The fertilization of heartsease 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 

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