THE INTEGRATION OF THE ORGANIC WORLD. 401 



we may still properly regard their respective lives as mutually- 

 dependent organic functions, as said in the preceding section. 

 We are enabled the better to see how the Earth's Flora and 

 Fauna, which are respectively accumulators of motion and 

 expenders of motion, form mutually-dependent parts of a 

 whole, and are in that sense integrated. And we shall be 

 prepared to see how all other relations between organisms 

 which make them subservient one to another, similarly con- 

 stitute elements in a general integration of the organic world. 



314J. Another form of mutual dependence and conse- 

 quently of integration is conspicuous that which accom- 

 panied the progressive increase of size in organisms of the 

 higher classes. We have but to contemplate the possibilities 

 to see that life must necessarily have commenced with minute 

 forms, and that the progress to larger ones must have been 

 by small steps. 



For had creatures of appreciable sizes been the first to 

 exist they would inevitably have disappeared from lack of 

 food. Having no resource but to devour one another, they 

 would quickly have brought life to an end. There must have 

 been smaller types serving as prey for larger ones before 

 these could continue to exist and to multiply: microbes 

 affording food to infusoria, infusoria affording food to such 

 sized creatures as the Entomostraca, these again supplying 

 food to small fishes, such as loch-trout, and these last yielding 

 to larger fishes masses sufficiently great for their needs : each 

 higher grade requiring lower grades of appropriate bulk. It 

 needs but to ask what would become of tigers if there were 

 no mammals larger than mice, to see that the animal world is 

 a linked assemblage, of which the connected members stand 

 within certain ratios of mass ; and that during the evolution 

 of higher and larger types the linking of grades has become 

 closer. 



That among plants considered as an aggregate relations 

 of like kind, though far less distinct ones, have all along 



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