228 A. G. HAMILTON. 
effect anything,”* The isolated productions of Australia on the 
other hand, have had uniform conditions and comparatively small 
range and so they cannot make way against those that have had 
such competition and range. 
‘In the same manner at the present day, we see that very many 
European productions cover the ground in La Plata, New Zealand 
and to a lesser extent in Australia and have beaten the natives, 
whereas extremely few southern forms have come to be naturalised 
in any part of the northern hemisphere, though hides, wool and 
other objects likely to carry seeds have been largely imported into: 
Europe during the last two or three centuries from La Plata, and 
during the last forty or fifty years from Australia.”+ Wallace 
says, ‘“‘'There is good reason to believe that the most effective 
agent in the extinction of species is the pressure of other species,. 
whether as enemies or merely as competitors.” { 
It is well known that few Australian plants have found a foot- 
ing in Europe notwithstanding the many facilities which commerce: 
offers for their introduction, and the few American weeds which 
have found their way to Europe do well only in the Mediterranean 
region. Even in New Zealand but a few Australian plants have. 
become naturalised as is shown by Mr. T. F. Cheeseman’s paper on 
the naturalised plants of Auckland (read before the Auckland 
Institute, November 1882). 
In America, the majority of introduced weeds are European, 
though at first they completely beat the natives, it is noteworthy 
that now the natives are holding their own, and even beating the. 
strangers, thus showing that competition has gone on long enough 
for some advantage to be gained by the natives. It is remarkable: 
too that the plants of Eastern America immigrated westward with 
man, and conquered the western plants at first ; but from a con- 
sideration of the facts the great American botanist Prof. Asa Gray 
was led to prophesy a return wave of western plants, and that is 
7 
| 
F 
now actually coming. 
* Op. cit., p. 319. + Op. cit., p. 340. 
t Island Life, p. 63. 
