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CHAPTERS ON EVOLUTION. 



It has been already remarked that "self-fertilisation" is the excep- 

 tion and " cross-fertilisation" the rule of plant-nature. At any rate, 

 the cases where " cross-fertilisation " is obviously the process which by 

 manifold contrivances nature seeks to further and effect, increase in 

 number year by year. Although self-fertilisation does occur, and 

 is a possibility even with normally cross-fertilised plants, yet the 

 whole drift of modern botanical teaching tends towards the recogni- 

 tion of the mutual interchange of pollen betwixt related flowers as the 

 normal way of plant-reproduction. Nor do the comparative results 

 to be hereafter detailed of cross and self-fertilisation in the least degree 

 vitiate these conclusions. On the other hand, every fact of botany 

 dealing with ascertained results of the one method of fertilisation, as 

 compared with those obtained by the other, testifies to the enormous 

 gain, possible and actual, to the plant- creation through the effects of 

 cross-fertilisation. The presence of so many different methods 

 whereby this end is secured, constitutes an eloquent fact in favour of 

 the supposition that the normal way of plant-life undoubtedly lies in the 

 direction of pollen-interchange, as a necessity for energetic develop- 

 ment and for the full fruition of the races and tribes of plants 

 that people earth's firmament. 



Within the limits of the present chapter it would be impossible to 

 enter into the discussion of those peculiarities of insect structure which 

 have been developed or modified in turn like the forms of flowers for 

 the due performance of the work of cross-fertilisation. It may suffice 

 at present to simply point out that the conformation of the legs of 

 certain insects, as well as the form of the mouth-parts, and even the 

 hairiness of body or the reverse conditions, all bear witness to special 

 adaptation in different insects for the fertilisation of special flowers. 

 Certain insects are known to confine their visits to special plants 

 some to one species of plant only; and probably, when this department 

 of the subject is more fully and completely studied, the number of 

 cases in which insect-visitation is of a rigid or exclusive kind, will be 

 largely increased. 



The two chief methods of cross-fertilisation, or, in other words, 

 of flower- fertilisation at large, are thus : (i) by insects, or more rarely 

 birds, snails, &c. ; (2) by the wind; whilst (3) pollen may be floated 

 on water from one plant to another, as in the case of Vallisneria 

 spiralis. Botanists term plants fertilised by insects " entomophilous," 

 and those fertilised by the wind " anemophilous." Some plants, e.g. 

 common rhubarb (Darwin), and some species of Plantago(Delpino and 

 H. Miiller) exhibit an intermediate condition, in that they may be fer- 

 tilised in either way. The wind-fertilised plants, as an " invariable 

 rule," according to Darwin, possess small and inconspicuous flowers, 

 whilst the insect-fertilised flowers, as might be expected, are conspi- 

 cuous, or, if not brightly coloured, are strong smelling. Moreover, 



