INTRODUCTORY ESSAY. XXV 
We thus observe that the want of an Arctic or Antarctic Flora at all in the Pacific islands, 
and the presence of an Arctic one in the American Alps, are the prominent features; and I shall 
confine my remarks upon these to the fact that, with regard to the isolated islands of the Pacific, 
they are situated in too warm a latitude to have had their temperature cooled by changes in the rela- 
tive position of land and ocean, so as to have harboured an Antarctic vegetation. With regard to the 
South American Alps, there is direct land communication along the Andes from Arctic to Antarctic 
regions; by which not only may the strictly Arctic genera and species have migrated to Cape Horn, 
but by which many Antarctic ones may have advanced northward to the equator*. 
There is still another point in connection with the subject of the relative antiquity of plants, and 
in adducing it I must again refer to the * Principles of Geology,” where it is said, “ As a general rule, 
species common to many distant provinces, or those now found to inhabit many distant parts of the 
globe, are to be regarded as the most ancient . . . . their wide diffusion shows that they have 
had a long time to spread themselves, and have been able to survive many important changes in 
Physical Geography." If this be true, it follows that, consistently with the theory of the antiquity 
of the Alpine flora of New Zealand, we should find amongst the plants common to New Zealand and 
the Antarctic islands, some of the most cosmopolitan; and we do so in Montia fontana, Callitriche 
verna, Cardamine hirsuta, Epilobium tetragonum, and many others. 
On the other hand, it must be recollected that there are other causes besides antiquity and 
facility for migration, that determine the distribution of plants; these are their power, mentioned 
above, of invading and effecting a settlement in a country preoccupied with its own species, and their 
adaptability to various climates: with regard to the first of these points, itis of more importance 
than is generally assumed, and I have alluded to its effects under Sonchus, in the body of this work. 
As regards climates, the plants mentioned above seem wonderfully indifferent to its effects $. 
Again, even though we may safely pronounce most species of ubiquitous plants to have outlived 
many geological changes, we may not reverse the position, and assume local species to be amongst the 
most recently created; for whether (as has been conjectured) species, like individuals, die out in the 
course of time, following some inscrutable law whose operations we have nót yet traced, or whether 
(as in some instances we know to be the case) they are destroyed by natural causes (geological or 
others), they must in either case become scarce and local while they are in process of disappearance. 
In the above speculative review of some of the causes which appear to affect the life and range 
of species in the vegetable kingdom, I have not touched upon one point, namely, that which concerns 
the original introduction of existing species of plants upon the earth. I have assumed that they have 
existed for ages in the forms they now retain, that assumption agreeing, in my opinion, with the facts 
elicited by a survey of all the phenomena they present, and, according to the most eminent zoolo- 
* Why these Antarctic forms have not extended into North America, as the Arctic ones have into South America, 
is a curious problem, and the only hypothesis that suggests itself is derived from the fact that though the Panama 
Andes are not now sufficiently lofty for the transit of either, there is nothing to contradict the supposition that they 
may have had sufficient altitude at a former period, and that one which preceded the advance of the Antarctic species 
to so high a northern latitude. : 
T Principles of Geology, ed. 9. p. 702. 
+ Mr. Watson (Oybele Britannica) gives the range of Callitriche in Britain alone as including mean tempe- 
ratures of 40? to 52°, and as ascending from the level of the sea to nearly 2000 feet in the East Highlands of 
Scotland. Montia, according to the same authority, enjoys a range of 36? to 52°, and ascends to 3300 feet; Epilo- 
bium, a temperature of 40? to 51°, and ascends to 2000 feet; Cardamine, a temperature of 37° to 52°, and ascends 
to 3000 feet. 
