lo6 COLh\ CLOCT'S C ALEX DA R. 



Asia, and still later of America, he has produced such a 

 series of changes in the native plants that many of them 

 have become at last pure weeds of cultivatit)n. There 

 arc some, like pimpernel and shepherd's-purse, that we 

 only know in this form ; they grow always on culti- 

 vated ground or waste patches, and their truly wild 

 types arc now utterl)- extinct and irrecoverable. None 

 are more peculiar in this respect than the weeds that 

 frc(jucnt cornfields and meadows ; and perhaps their 

 most marked peculiarity is their exact synchronism with 

 the grass or the wheat among which they grow. All of 

 them spring uj) together, flower together, and ripen their 

 seeds together. They are cut down with the crops ; 

 their seeds are sown with the crops ; and they are 

 carried to all parts of the world with the seed-corn and 

 the grasses. At first sight people are inclined to say 

 that this is pushing a true principle too far : cultivation, 

 they think, has existed on the earth for so short a period 

 that natural selection has not yet had time to act ui)on its 

 concomitant weeds. They might almost as well object 

 to an account of a shipwreck in which only the best 

 swimmers escaped, on the ground that in those few 

 minutes natural selection would not have time to single 

 out the bravest muscles and the strongest thews. 

 There are circumstances in which the selection is abso- 

 lute and instantaneous — as, for example, in prairic-fires 

 or submerged islands. The annual cutting of the corn 

 and the grasses acts almost as absolutely and effectively. 

 From year to year, at a relatively fixed date, every plant 

 \w vast tracts of cultivated country is cut down and 

 carried away from the fields. Most of these plants are 

 peculiar to the tilth of the lowlands ; they are different 



