226 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



determination of plant-structure, but motion has also 

 had an important influence. The motion, however, 

 has originated in small degree in the plant itself, but 

 has been derived from without. Some importance 

 must be ascribed to the effects of winds, but the prin- 

 cipal source of the especial strains to which plants 

 have been subjected, has been the insect world. In- 

 sects have been inhabitants of land-plants since their 

 origin in early Paleozoic ages, and the mutual relations 

 of plants and insects have ever been intimate. As has 

 been insisted by Miiller and Henslow, the uses to 

 which the floral organs have been put by hymenopte- 

 rous and other insects have been probably a principal 

 cause of the forms assumed by the former. From this 

 direction has been derived the kinetogenetic influence 

 in plant evolution. The few independent movements 

 displayed by plants may have had some influence on 

 the evolution of their structure. We have no reason 

 as yet to suppose that such movements have any other 

 than purely physical factors. 



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