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THE AMERICAN NATURALIST [Vol. XLII 



field that give promise of the solution of the problem of 

 species. 



Perhaps no phenomena bring the principles just 

 enunciated into more clear relief than do those of natural 

 selection. Many species must be born that never have 

 an opportunity to survive, owing to their lack of adapta- 

 tion to the surroundings in which they originate. The 

 mutants of (Enothera lamarckiana, though developed 

 under essentially similar conditions, do not appear 

 equally adapted to the environment in which they first 

 appeared ; had they been left to themselves, some mutants 

 would have perished, while others (and perhaps espe- 

 cially (Enothera gigas) might have lived. The citrus 

 hybrids, developed in government experiments in Florida, 

 form another group of new forms, some of which are 

 best adapted to one climate, and others to another. Since, 

 therefore, no necessary adaptive relation must exist be- 

 tween a new species and the region of its birth, it is clear 

 that the laws of selection determine the success or failure 

 of new species. The interpretation of selection is a field 

 problem, an ecological problem. A theoretical plant 

 species may be produced in the laboratory, but the real 

 species that make up the vegetation of the world are de- 

 veloped and must be studied out of doors. 



One of the noblest aims of ecology is the destruction 

 of many of the " species" of our manuals. Where the 

 critical study of species is confined to the herbarium, it 

 often happens that ecological varieties or habitat forms 

 are given specific rank. An excellent instance of this 

 is seen in the case of Polygonum amphibium and P. 

 hartwrightii. The latter, which looks wonderfully dif- 

 ferent from the former in herbaria, can be developed at 

 will by growing P. amphibium on land instead of in the 

 water. Not infrequently a plant may be found on the 



commonly be ' referred to P. amphibium, and aerial 

 branches that would be regarded as P. hartwrightii. 

 Bonnier 's classic experiments, whereby many alpine 



