922 ORIGIN OF THE 
geographical plant-distribution, such an hypothesis becomes extremely 
improbable, and in some cases quite untenable. In order to establish 
the idea of common centres for the creation of species, we must 
be prepared to prove by what means these have wandered abroad. Tt 
will be easily seen that, while in many cases these means appear to 
have been adequate for the end, there are many others utterly insuffi- 
cient to account for the existence of the same species in far distant 
countries. The ordinary means are the following: man, who by 
his occupations and pursuits, purposely or otherwise, carries plants 
from one place to another; the tides of the sea, transporting fruits 
(Cocoa-nuts for instance) from one coast to another; rivers, which 
convey fruit and seeds of alpine plants into valleys; winds, that waft 
them, especially such as are endowed with hairy or feathery appen- 
dages, or so-called wings, contributing to their easy transport; birds, 
who occasionally perform their part in this operation. We may, 
moreover, assume, where geographical obstacles interpose themselves, 
that tracts, which in former times connected countries, have sunk (the 
Channel, the Mediterranean, &c.). But it will be easily seen that these 
means are very inadequate, when we consider that many species are 
common to the Alps and Pyrenees on the one hand, and the Scan- 
dinavian and Scotch mountains on the other, without being found on 
the intermediate plains and hills; that the flora of Iceland is nearly 
identical with that of the Scandinavian mountains; that Europe and 
North America, especially their northern parts, have various plants in 
common, which have not been communicated by human aids. Still 
greater and almost insurmountable obstacles to such a mode of explain- 
ing things arise, from the fact that there are plants in the Straits of 
Magellan and on the Falkland Islands, which belong to the flora of 
. the arctic pole,—for instance, PAJeum alpinum and Erigeron alpinus ; 
and that several European plants appear in New Holland, Van 
. Diemen’s Land, and New Zealand, which are not found in the in- 
= tervening tropical countries, nor are likely to have been introduced ; 
= which is strikingly instanced in the case of several fresh-water 
plants, such as our common Reed, Alisma Plantago, several species 
of Scirpus and Lemna, Typha, Aira flexuosa. The quotation of 
‘these species, common both to the arctic and antarctic flora, is 
not derived from those periods when species were not so rigor- 
ously defined as in our modern times. The most recent researches, 
