FLORAL BIOLOGY. 209 



tion of highly specialised cross-fertilising flowers is largely 

 prevented by the lack, in new districts, of insects suited to 

 them. Cross-fertilised wind flowers, on the other hand, 

 are often widely scattered, and so are the plants of many 

 orders such as Compositse, whose mechanism is very effective 

 for cross-fertilisation, but not specialised for too narrow a 

 group of insects. To say, as many writers from Axell 

 onwards have said, that self-fertilised flowers are the highest 

 type, and best fitted to survive in the struggle for existence, 

 is to oro as much too far towards one extreme as the earlier 

 workers went towards the other. 



MacLeod makes a tentative grouping of flowering 

 plants into two classes — "capitalists" and ''proletarians". 

 The first group is composed of plants which have large 

 reserve stores and can afford to pay for cross-fertilisation 

 by producing large and attractive flowers, etc., while still 

 retaining enough material to supply the seeds. Such are 

 our trees and shrubs, most perennial herbs and many 

 annuals, whilst the second group is composed chiefly of 

 annual and ephemeral plants whose supply of material is 

 all or nearly all used for seeds, and which therefore, being 

 unable to pay for crossing, are self-fertilised. Such are 

 Alchemilla arvensis, Scleranthus animus, Radiola hnoides. 

 The idea is suggestive and brings out a point to which too 

 little attention has been paid (though Darwin saw its im- 

 portance), viz., the great advantage to a plant of having its 

 seeds provided with a large supply of reserves, thus ena- 

 bling them to germinate rapidly and get the start in the 

 struggle for light and air. The few data (2) we possess 

 upon the advantages of heavy seeds as against light ones 

 go to show that the advantages of the former are at least 

 as great as the advantages of crossing, so that it might well 

 pay a plant to fertilise itself and produce heavier seeds 

 rather than try to obtain cross-fertilisation. Of course there 

 is here also a balance to be struck between conflicting ele- 

 ments. The heavier the seed the less its chances of dis- 

 tribution. Again, in the case of annuals, where the seeds 

 are liable to germinate in autumn and be killed by the frost, 

 it becomes advantageous to produce great numbers of seeds 



