June 10, 1910] 



SCIENCE 



897 



ment but that these rapidly and progres- 

 sively become less and leas with each step 

 in advance. This is perhaps true as it is 

 a general law of nature. Yet the improve- 

 ment made in some .supposed cases is 

 vastly greater than could possibly have 

 been anticipated. Thus the gap from 

 Johnson grass to its supposed derivations, 

 such as Kafir, Jerusalem com, milo. 

 Sumac sorghum and a host of other vari- 

 eties LS so great as to stagger one's belief. 

 Yet the botanical evidence is good enough 

 to warrant critical experimental investi- 

 gation. 



How much further wheat, corn and other 

 long-cultivated plants may still be im- 

 proved can not be foretold, because we are 

 too ignorant of the potentialities which 

 have brought them to their present devel- 

 opment. In any attempt that may. be 

 made to redevelop the cultivated forms 

 from the wild forms, two things will have 

 to be considered— fir.st, that various forms 

 of the wild plant may and probably do 

 exist in different regions— and second that 

 even beginning with the same wild form its 

 descendants in different regions will prob- 

 ably vary in different directions. Only on 

 one or both of these hypotheses can we 

 explain the fact that with anciently culti- 

 vated plants each region has its own pe- 

 culiar varieties and types. The problem 

 of the origin of the more marked varieties 

 of the plants cultivated in and since pre- 

 historic times becomes an exceedingly com- 

 plex one, probably capable of being dupli- 

 cated only in small part. "We must not 

 underestimate the ability of even very low 

 races of agricultural people to improve 

 their cultivated plants. Certainly the 

 Indians developed corn to a very high 

 degree and had some pretty clear ideas 

 regarding its culture. For example, the 

 Virginia Indians made it a point to plant 



in each hill seed from several different 

 ears. 



It seems to me that we too often err on 

 the side of making phenomena appear more 

 simple than they really are. Plants are 

 vastly more complex organisms than our 

 formulated ideas recognize. Many of their 

 phenomena completely baffle us. For ex- 

 ample, I might mention what has been 

 called aggressiveness in a plant — namely, 

 its ability not only to occupy and maintain 

 the soil, but to .spread and crowd out other 

 plants. This ls particularly evident in. 

 plants introduced from one country to 

 another. Thus nearly all of our weeds are 

 of old world origin. The same is true of 

 our permanent meadow and pasture plants, 

 where ability to occupy and hold the 

 ground against weeds is &s.sential. In this 

 respect our American grasses and clovers 

 utterly fail before the foreign immigrants. 

 Some other striking instances of the great 

 aggressiveness of an immigrant may be 

 cited. The introduced English violet is 

 said to be the worst of weeds in Mauritius ; 

 American cacti are becoming a pest in 

 South Africa; the marvelous vigor and 

 spread of the American waterweed (Elo- 

 dea) under European conditions is well 

 known. Several explanations of these and 

 similar phenomena have been advanced. 

 The commonest one is that the plant is in- 

 troduced but its fungous and insect ene- 

 mies are not. Therefore, the plant is re- 

 leased from all handicaps as it were and 

 can exercise to the utmost its inherent 

 energy. A second and related explanation 

 is that every plant becomes held within 

 limits by the competition of other plants 

 in its native land, and very often in the 

 new environment the native plants do not 

 have an equal r&straining influence— be- 

 cause they have had to contend with a 

 different set of competitors. A third idea 

 is that any organism with the ability to 



