104 COLIN CLOUT'S CALENDAR. 



them spring up 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 tiad time to act upon 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 absolute and 

 instantaneous as, for example, in prairie-fires or sub- 

 merged 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 in 

 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 in type 

 both from the woodland flowers and from the hedgerow 

 weeds. Hence their only chance of survival is by ex- 

 actly adapting their own habits to those of the food- 

 plants among which they dwell. 



In the beginning, no doubt, they varied greatly in 

 their periods of development ; some were earlier and 

 some later. But every weed which ripened its seeds too 

 late would naturally be cut down green, so as to perish 

 utterly ; while every weed which ripened them too early 

 would stand a fair chance of having them buried beneath 

 a whole sod's thickness of ploughed land. Thus only 

 those which happened exactly to tally in time with the 

 corn or the grasses would succeed on an average in keep- 



