78 



the am Fine ax naturalist 



[Vol. XLII 



lation. Doubtless these species in duration and in nature 

 correspond more nearly to genera or families of higher 

 animals than to actual species. Perhaps minor specific 

 differences such as we note among arthropods or verte- 

 brates are intangible or non-existent. The effects of iso- 

 lation may be tangible only among forms which possess 

 more varied relations with their environment. 



The application of this law to plants has also been 

 denied. But geminate species are just as common in 

 botany as in zoology, and the effects of isolation in species- 

 forming are just as distinct. The law is just as patent 

 in the one case as in the other. It is merely obscured by 

 other laws or conditions which obtain among plants. 



In the nature of things, most physical barriers are more 

 easily crossed by plants than by animals. The possibili- 

 ties of reinvasion are thus doubtless much increased. The 

 plant is limited by climate, rainfall, nature of soil, and the 

 same species is likely to occupy all suitable locations 

 within a large area. Animals are more mobile than plants 

 within their range, a fact which tends to keep the in- 

 terbreeding masses more uniform. In the struggle for 

 existence, the plant is pitted against its environment. 

 Whether a plant survives or not depends not much on 

 the nature of the seed, but mainly on its relation to the 

 spot on which it falls. There is little selection within the 

 species due to the choice of one individual as against 

 another. This can only happen where plants are over- 

 crowded, and there the survival is mainly that of the 

 seed whose roots run deepest. There is little room for 

 struggle between closely related species. Each individual 

 grows— if it can— on the spot where it falls. The vari- 

 ations among plants are great, but these variations are 

 mostly lost unless reinforced by segregation. There is 

 no likelihood of the survival of DeVries' mutants of the 

 evening primrose if these forms are left free to mix in 

 the same field. 



Among plants we often notice the fact— rare though 

 not unknown among animals— of numerous species of the 

 same genus occupying the same area. In some cases these 



