10 ORGANIC EVOLUTION 



near Port Henderson, Jamaica. Among the shells there col- 

 lected were sixty-eight distinct varieties, as indicated by the 

 color and the pattern of their markings. I know of no finer 

 example of variation in color and color pattern than is 

 shown in these little shells. 



Remembering now these facts of heredity and variation, 

 let us observe the conditions under which organisms live, and 

 see how these operate to cause and guide their evolution. 



The struggle for existence. 



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As we go about unobservant through the woods and 



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fields, glancing carelessly at the bright Mowers and the birds 

 busily seeking their food or singing in apparent contentment, 

 or as we look over the ocean and think of the fish darting 

 swiftly through the clear water, it all seems to us an idyl 

 of perfect happiness, full of ease and play. We rarely think 

 of the constant struggle for food and life in which all these 

 trees and flowers, all the fish and birds and other animals, 

 are engaged. We fail to see that for them life is one 



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continual struggle ; that the gathering of food, that resist- 



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ance to the unfavorable conditions of climate, cold, drouth, 

 flood, and storm, that rivalry in marriage and the effort 

 to rear their young when born, absorb the energy of animals 

 and plants alike ; and that, despite the strenuous efforts 

 put forth, the result, in the great majority of cases, is 

 failure and death. Yet this is by far the truer picture 

 of organic nature. Everywhere is starvation and death, 

 failure to reach success in their own lives or in rearing 

 their young. To some this aspect of nature may not 

 seem so pleasant to contemplate, yet a moment's consid- 

 eration will show its truth. 



