INTRODUCTORY ESSAY. 
the scarce old forms, and a strong ally of those already common : nor can it be doubted but thai 
many of the small local genera of Australia, New Zealand, and South Africa, will ultimately disappear, 
owing to the usurping tendencies of the emigrant plants of the northern hemisphere, energetically 
supported as they are by the artificial aids that the northern races of man afford them. 
On some of the Naturalized Plants of Australia. 
My sources of information upon this subject are unfortunately extremely scanty, and almost con- 
fined to data procured from the vicinity of Melbourne, where Mr. Adamson has paid especial atten- 
tion to the introduced species which have run wild, and assumed the positions and importance of 
native plants. It would be interesting to discover the date and particular circiinc 
which these plants were introduced, and so to register their increase and migrations as to afford 
to succeeding observers the means of comparing their future condition with their present. In the 
early times of a colony, there is comparatively little difficult) in distinguishing the colonists from 
spots and eject the native, or, taking their places by them, appear, like them, to be truly indigenous. 
There are many interesting subjects of inquin connected with this replacement of one vegetable by 
another, such as determining the relation - between the facility with which the new plants of certain coun- 
tries or genera are introduced and establish themselves, and the countries such plants come from, or 
Floras of which they form a part* Much of course depends on the new comer finding a suitable 
climate and soil for its future increase, but there may be more in the physique or constitution of the 
new comer that enables it to displace other plants which are apparently equally well (if not better) 
adapted to the circumstances it finds itself environed with. The nature of the past intercourse be- 
tween Europe and Australia should lead us to anticipate that a far greater number of English plants 
arc naturalized in Australia than of Australian in England ; but the fact of importation does not 
explain naturalization, nor how it is that no Australian plant has become naturalized in England. 
This total want of reciprocity in migration is no doubt mainly attributable to climate, but then we 
have the apparent double anomaly, that Australia is better suited to some Knglish plants than Eng- 
land is, and that some English plants are better suited to Australia than those Australian plants were 
which have given way before English intruders. Pof my own part. I am i- 
the three elements of (1) abundant exportat;* 
and horticultural purposes, and scanty export of Australian seed produce t 
adaptation of Australia than England to support numerous forms of vegetable life; and (3) abun- 
dance of unoccupied ground in Australia as compared with England ; are, combined, all but sufficient 
to account for the predominance of so many Kur.-pean naturalized plants in Australia, and for the 
converse state of things in England. But I think it may still remain to be seen whether the altered 
* The reader will find some admirable discussions on this and kindred Sttbjects is the chapters of A. De Can- 
dolle's £ G-eographie Botauique Raisonnee ' ci 
