40 
POPULAR SCIENCE REVIEW. 
some time previously, but that, from the ploughing and crop- 
ping of the soil, they had not had a fair chance of developing 
flowers. 
The facts we have mentioned are, in the main, intelligible 
enough. W e can see the why and the wherefore without much 
difficulty ; but it is not so always. For instance, it is difficult 
to account for the signal defeat that native plants often incur 
at the hands of invading strangers. 
Why does the water-cress, harmless enough in our ditches, 
block up the water-courses in New Zealand to such an extent 
as to become a costly nuisance ? What can there be in Eng- 
lish ditches and canals so propitious to the growth of tlie 
American water- weed (^Anacharis) as to have caused it to 
obstruct even our navigable rivers ? In America, whence it 
came, it is no more of an inconvenience than any other water- 
weed. Why in other places does the white clover [Tri folium 
re'pens') overcome the native grasses, and dispossess them of 
their territory ? Why has a particular grass, the Stipa tortilis, 
invaded the South Eussian steppes to such an extent as to dis- 
place almost every other plant ? 
There are numberless such instances — from that afforded by 
the island of St. Helena, in which the original vegetation is 
almost completely dispossessed, and its room occupied by 
foreign importations, to the banks of a Surrey river, yellow 
with the flowers of an American balsam — and the reason is not 
obvious. The fact is patent, and is not without analogies in the 
virulence with which epidemic diseases spread when introduced 
for the first time among a population not heretofore subjected to 
them. 
Such cases as these recall the opinions of Humboldt and 
others on the antipathies of plants. According to this notion 
certain plants are positively injurious to others, not so much by 
any peculiarity of structural organisation as by the excretion of 
matters hurtful to other plants. It has been asserted, for 
instance, that the darnel (^Lolium temulentum) is injurious to 
wheat ; that a species of thistle {Servatida arvensis) is obnoxious 
to oats ; that a spurge {Eupliovhia Peplus) and a scabious 
(Knautia arvensis) are detrimental to flax ; and spurrey (^Sper- 
(jula arvensis') similarly prejudicial to buckwheat. 
In so far as this detrimental influence is due to any excre- 
mentitious product from the plant, the verdict given by modern 
physiologists amounts to “ not proven.” Some would even say 
“ not guilty ; ” but we do not see clearly how those who take 
this view can reconcile it entirely with the existence of that 
natural alternation of which Dureau de la Malle speaks, and 
which is admitted by all subsequent observers. 
Mere exhaustion of the soil will not account for the 
