156 NEW ZEALAND PLANTS. 
required for milling purposes are weeds, except such as benefit the 
association, or a forest occupying ground of greater value for grazing 
than for the growing of timber is a distinct weed-association. 
On the other hand, a so-called “ noxious weed’? may be no 
weed at all. Where the unploughed ground close to the hedge of 
a paddock of lucerne in Marlborough is occupied by Californian 
thistle (Cnicus arvensis) the latter is certainly not a “ noxious 
weed,” or, indeed, a weed at all, for it is unable to extend for 
even one inch into the lucerne; and in that position it is neither 
wanted nor unwanted—it does, in fact, neither harm nor good. 
Foxglove is certainly a bad weed in certain parts of the Marl- 
borough Sounds area, where the land it occupies could be profit- 
ably farmed; but for a borough situated in an area where 
it is impossible for this plant to spread to proclaim it a noxious 
weed which must be eradicated, when there could be none to 
eradicate, was not a wise measure. On the other hand, as a 
garden-plant the foxglove could do no harm. 
Such well-known weeds as the sorrels, docks, fat-hens, and thistles 
would in the original primeval world each have its proper place in 
the primitive plant-association to which it might belong, and would 
be present in no extraordinary numbers. It was the changes brought 
about by cultivation in its various forms, fires, and the close grazing 
of domestic animals which upset the balance of nature. Then those 
plants whose structure and habits were most in harmony with the 
changed conditions would become more numerous at the expense of 
those not so well equipped, and as the conditions antagonistic to the 
plant-association as a whole increased, so would the species best suited 
to such conditions likewise increase. All this is to say that many 
plants are potential weeds, ready to become active ones as soon as 
suitable conditions arise. Thus, through the many centuries of culti- 
vation in Europe, aided by the ever-expanding intercourse with other 
lands, the great army of now almost cosmopolitan weeds has been 
gathered together—the very pick of the vegetable world in temperate 
climes for thriving under the artificial conditions imposed by man. 
It is the old story that when one interferes with nature she exacts 
remorselessly her tribute. 
It is only in a virgin vegetation that the evolution of a weed 
can actually be witnessed. The New Zealand flora has furnished 
some rather striking examples. Some of these cases have been 
