XIV FLOKA OF TASMANIA. 



17. 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 

 temperate. 



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 land 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 

 connection. 



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 ckeaacters, 

 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, where many are present, and where those of low morphological importance may have a compa- 

 ratively high physiological significance. 



