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THE AMERICAN NATURALIST. [Vol. XXXVI J- 



elementary characters of any species of a higher plant may be 

 reckoned at a few thousand — about 6000 in CEnothera. If 

 Lord Kelvin's estimate of the period during which life has 

 existed on the earth is accepted it might be concluded that in a 

 general way the average interval separating mutable periods of 

 any plant must be several thousand years, although nothing 

 in the nature of the question may be taken to indicate anything 

 like uniformity in the matter. Some writers have put forward 



or twenty-five hundred million years, would be necessary for the 

 derivation of the existing forms of plants and animals by natural 

 selection. It must be admitted that both ideas are valuable 

 chiefly as attractive examples of imaginative grasp rather than 



It will be recalled that the various theories which have been 

 put forward to account for the origin of species have been held 

 by their authors and advocates to be mutually exclusive, and it 

 seems to have been, and is still taken for granted by the major- 

 ity of writers, that all organic forms, both plants and animals, 

 have arisen in the main by one simple method of biological pro- 

 cedure. The development of biological science has certainly 

 reached a stage where this a priori generalization may well be 

 abandoned. I can not say that a candid review of the mechan- 

 ism of protoplasm, or of the pertinent evidence, from any point 

 of view compels adherence to this ancient assumption. 



The great amount of critical study that is being directed to 

 the study of hybrids and hybridization is widening the horizon of 

 this subject momentarily, and the result of our recently acquired 

 information leads us to conclude that species may originate by 

 crossing. In such instances the new types are due either to new 



strated source of origin of species however, and it is becoming 

 more and more generally recognized that more than one method 



