ON THE HYPOTHESIS OF EVOLUTION. 145 



That it has been to a large extent the means of preservation of 

 those structures known as specific, must, I think, be admitted. 

 They are related to their peculiar surroundings very closely, and 

 are therefore more likely to exist under their influence. Thus, if 

 a given genus extends its range over a continent, it is usually 

 found to be represented by peculiar species — one in a maritime 

 division, another in the desert, others in the forest, in the swamp 

 or the elevated areas of the region. The wonderful interdepend- 

 ence shown by Darwin to exist between insects and plants in the 

 fertilization of the latter, or between animals and their food- 

 plants, would almost induce one to believe that it were the true 

 expression of the whole law of development. 



But the following are serious objections to its universal appli- 

 cation : 



First : The characters of the higher groups, from genera up, 

 are rarely of a character to fit their possessors especially for sur- 

 rounding circumstances ; that is, the differences which separate 

 genus from genus, order from order, etc., in the ascending scale 

 of each, do not seem to present a superior adaptation to surround- 

 ing circumstances in the higher genus to that seen in the lower 

 genus, etc. Hence, superior adaptation could scarcely have 

 caused their selection above other forms not existing. Or, in 

 other words, the very differences in structure which indicate suc- 

 cessional relation, or which measure the steps of progress, seem to 

 be equally well fitted for their surroundings. 



Second : The higher groups, as orders, classes, etc., have been 

 in each geologic period alike distributed over the whole earth, 

 under all the varied circumstances offered by climate and food. 

 Their characters do not seem to have been modified in reference 

 to these. Species, and often genera, are, on the other hand, 

 eminently restricted according to climate, and consequently vege- 

 table and animal food. 



The law of development which we seek is indeed not that 

 which preserves the higher forms and rejects the lower after their 

 creation, but that which explains why higher forms were created 

 at all. Why in the results of a creation we see any relation of 

 higher and lower, and not rather a world of distinct types, each 

 perfectly adapted to its situation, but none properly higher than 

 another in an ascending scale, is the primary question. Given 

 the principle of advance, then natural selection has no doubt 

 modified the details; but in the successive advances we can 

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