XIV FLORA OF TASMANIA. 
1 7. Though we rarely find the same species running into the same varieties at widely sundered 
localities (unless starved or luxuriant forms be called varieties), yet we do often find a group of spe- 
cies represented in many distant places by other groups of allied forms ; and if we suppose that indi- 
viduals of the parent type have found their way to them all, the theory that existing species have 
originated in variation, and that varieties depart further from the parent form, will account for such 
groups of allied species being found at distant spots; as also for these groups being composed of 
representative species and genera. 
18. No general relations have yet been established between the physical conditions of a country 
and the number of species or varieties which it contains, further than that the tropical and temperate 
regions are more fertile than the polar, and that perennial drought is eminently unfavourable to 
vegetation. It is not even ascertained whether the tropical climates produce more species than the 
19. Though we cannot explain the general relations between the vegetation and physical condi- 
tion of any two countries that contrast in these respects, we may conclude as a general rule that 
those tracts of laud present the greatest variety in their vegetation that have the most varied combi- 
nations of conditions of heat, light, moisture, and mineral characters. It is, in the present state 
of our knowledge, impossible to measure the amount of the fluctuations of these conflicting con- 
ditions in a given country, nor if we could can we express them symbolically or otherwise so as to 
make them intelligible exponents of the amount of variety in the vegetation they affect; but the fol- 
lowing facts in general distribution appear to me to be favourable to the idea that there is such a 
There are certain portions of the surface of the globe characterized by a remarkable uniformity 
in their phsenogamic vegetation. These may be luxuriantly clothed, and abound in individuals, but 
are always poor in species. Such are the cooler temperate and subarctic lake regions of North 
America, Fuegia and the Falkland Islands, the Pampas of Buenos Ayres, Siberia and North 
Russia, Ireland and Western Scotland, the great Gangetic plain, and many other tracts of land. 
Now all these regions are characterized by a great uniformity in most of their physical characters, 
and an absence of those varying conditions which we assume to be stimulants to variation in a loca- 
lity. On the other hand, it is in those tracts that have the most broken surface, varied composition 
of rocks, excessive climate (within the limits of vegetable endurance), and abundance of light, that 
the most species are found, as in South Africa, many parts of Brazil and the Andes, Southern 
France, Asia Minor, Spain, Algeria, Japan, and Australia. 
20. The Polar regions are chiefly peopled from the colder temperate zones, and the species from 
the latter which have spread into them are very variable, but only within comparatively small limits, 
particularly in stature, colour, and vesture. Many of these polar and colder temperate plants are 
also found, together with other species closely allied to them, on the mountains of the warm tempe- 
rate, and even tropical zones; to which it is difficult to conceive that they can have been transported 
by agencies now in operation. 
21. The Floras of islands present many points of interest. The total number of species they 
contain seems to be invariably less than an equal continental area possesses, and the relative numbers 
of species to genera (or other higher groups) is also much less than in similar continental areas. 
The further an island is from a continent, the smaller is its Flora numerically, the more 
of organs in plants, " 
ratively high phy: 
