16 Transactions of the [Sess. 



inaritima, aud Plautago maritima growing at the sea-shore, and 

 then, after an interval, on bleak mountain-tops, is not an easy task. 

 It cannot be said that the conditions of existence at alj^ine limits 

 are those next favourable for these plants to those conditions in 

 which they live at the sea-shore. Nor do we think the hypothesis 

 that these plants were once universally distributed between the two 

 spots is correct, for it does not seem to us consistent to think that 

 a plant about to be run down in the struggle for existence could 

 retreat to two habitats so entirely distinct. 



A large number of instances could be given in which we find that 

 the species belonging to a single genus occupy different habitats. 

 Thus in the genus Carex we have species living on the sea-shore, 

 the marsh, and dry ground. The genus Veronica has species to 

 be found in marshy, semi-marshy, dry, and mountainous ground. 

 Other instances will occur to every one. How is it that two plants, 

 the differences between which the botanist only can determine, 

 occupy two habitats so entirely distinct from each other? Should 

 not two species so similar to each other be able to live one in the 

 habitat of the other? Take two common plants, Veronica Becca- 

 bunga and V. hedera?folia : will the difference between them in the 

 matter of leaf-form, or minute difference in the flower, account for 

 the one growing in the marsh and the other on the wayside ? We 

 are inclined to think that it will not. It is a fact that it is a 

 difScult matter to acclimatise alpine plants in gardens ; and it has 

 been proved that the best means of doing so successfully is to save 

 seeds from those alpines which have previously managed to thrive. 

 It is found that the plants which spring from these seeds are better 

 fitted to grow well than the plants from which they were derived. 

 A similar process may go on in nature. Suppose we have two 

 marshes where one plant grows. Suj)pose also that the one spot 

 very gradually becomes dry land, either through the gradually 

 filling up of the marsh by its own decayed vegetation, or by the 

 drainage, owing to some upheaval, being altered. As we have 

 supposed the change to proceed slowly, there is every reason to 

 think that the plant in question will be able to thrive until the 

 character of its former habitat is entirely changed. Differences in 

 the flower may come about by the unconscious selection by means 

 of insects which have not before visited it. During all this time 

 the same plant may be growing in the marsh which was not in a 

 process of transition into dry land. There is no doubt but that the 

 large number of species belonging to one genus that live under 

 identical conditions may be owing to the variation of one or two 

 original species, through the influence of the struggle for existence. 

 But it seems to us that when we find two species nearly identical 

 with each other inhabiting spots entirely different in character, the 

 cause of this must be owing to some such process as that just 



