274 AGRONOMY 



may have given them some advantage over their competitors 

 and helped them to hold their lead in the race. Those less 

 able to carry on the struggle ultimately and inevitably perish. 

 Thus there is in nature a constant and widespread selection 

 of the best, quite akin to that which man exercises, with this 

 difference, that in natural selection the plants are steadily 

 adapted to their habitats and the species and varieties kept up 

 to standard ; while in artificial selection, such as man prac- 

 tices, the aim has been to produce better strains for certain 

 purposes without regard to the ability of the plant to survive 

 in the struggle with other plants, since cultivation relieves the 

 plant of much of this struggle. This explains why our garden 

 plants are so easily overcome by weeds. They have been cul- 

 tivated so long, that they have lost the power to take care of 

 themselves. The weeds, on the other hand, have been de- 

 veloped into most efficient plants both by nature and by the 

 hand of the gardener. Only the most resistant could withstand 

 the two. 



Results of variation. Without variation evolution would 

 be at a standstill and no possibility of improvement would 

 exist. The tendency of plants to constantly vary in different 

 directions has saved whole races from extinction, while the in- 

 ability to change with changing conditions has as certainly 

 caused the death of many others. Plants are never perfectly 

 adapted to their habitats, but variation enables them to fit 

 into the plant covering of the region with the least friction, 

 while nature constantly weeds out the unfit. Usually the 

 changes in plants are cumulative, and, given sufficient time, 

 two plants nearly alike at the beginning may ultimately come 

 to be very different if exposed to different conditions. It is 

 not difficult to see that all the plants which now inhabit the 

 earth may have been derived from a single primitive form 

 through long ages of variation. This explains the existence 

 of many plants of the same general type they have been 



