12 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



venture on another induction. The sub-tropical climate 

 of any continent is apt to be desert. Immediately above 

 the desert, -where there is more rain, the flora is much 

 influenced by the very hot summer which becomes the 

 resting time for vegetation. The scorching wind withers 

 the herbage, at the same time ripening its seeds. Here 

 is the region of annuals. Here man found the large- 

 grained grasses which he cultivated for food, and with 

 them he was bound to favour the plants useless to himself 

 which dwelt with the corn and ripened their seeds along 

 with it ; and forcing the corn to grow in other climates, 

 he opened the ground for weeds, which only very careful 

 cultivation can exterminate, and the least carelessness 

 may re-introduce or spread to other regions. 



Such was the action of man down to the threshold of 

 our own times. The same in kind though increasing in 

 amount and thoroughness, the cultivation of the land 

 tended steadily to the replacing of the truly wild flora 

 by a weed flora, the exact constitution of which depended 

 on the amount of disturbance of the soil. In the fields 

 the weeds are mostly annual, on the roadside and hedges, 

 mostly perennial or biennial. I admit that the problem 

 of weed-distribution and of the selection by wdiich any 

 particular species is enabled to take its place in the w r orld 

 of weeds is very complex. I have, however, given 

 instances which may assist towards a solution. But a 

 still more interesting problem awaits me. 



Hound any big village or country town the truly wild 

 flowers gradually disappear as country merges into 

 suburbs and suburbs into streets. In a fairly pure 

 atmosphere there is but one obvious cause of this — human 

 pedestrian traffic, but this is not very important. It is 

 true that the roads are dusty and some plants cannot stand 

 dust, but most of them scarcelv mind it, the rain washes 



