THE FACTORS OF DISTRIBUTION 8r 



plants being thus comparatively rare, and the whole 

 flora, owing to the less intense struggle for existence, 

 having been, perhaps, less forced to adapt itself to the 

 environment, annual weeds, or other introduced plants, 

 are often able to establish themselves with ease. They 

 may spread at the expense of the indigenous or earlier 

 flora. Thus the Dutch Clover (Trifolium repens L.) 

 competes with the far larger New Zealand Flax (Phor- 

 mium tenax Forst.): 269 introduced species in Mauritius,, 

 out of a total of 705, are largely exterminating the 

 native plants; and in St. Helena introduced plants are 

 doing even more than goats towards the extinction of 

 the native flora. 



The summits of mountains in islands of warm regions 

 do not possess a true Alpine flora as do those of the 

 continent. They are mostly characterised by a pro- 

 fusion of Ferns, and by the mere dwarfing of the species 

 of the lower regions. This is explained by the absence 

 of that ebb and flow of northern species under the 

 advance and retreat of the Glacial Period which mostly 

 affected the continents. 



A comparative scarcity of flying insects to pollinate 

 them, and the concomitant scarcity of conspicuously- 

 coloured flowers, are said to be a feature of insular 

 floras, whether continental or oceanic, the prevalence of 

 high winds being supposed to explain the fact. In this, 

 and in other characters, the distinction between con- 

 tinental and oceanic islands would seem, like that 

 between insular and continental climates, to be one of 

 degree rather than one of kind. Volcanic islands near 

 the mainland may bear flowers differing no more from 

 those of the mainland than do the floras of those detached 

 areas of sedimentary rocks which we term continental. 

 There is, in fact, a gradation in the proportion of an 

 endemic element to the whole flora. So, too, what have 

 been termed " harmonic " and " disharmonic " dis- 

 tributions graduate into one another, the former showing 

 that development of large groups with many species 

 characteristic, on the whole, of continents, while the 

 latter shows gaps in series and many monotypic groups, 

 the result of occasional introduction as opposed to 

 wholesale migration. 



The illustration of some of these principles will be 

 found later on in the descriptions of some insular floras. 



F 



