182 THE EVOLUTION THEORY 



How near Sprengel was to reaching a complete solution of the 

 problem is now plain to us, for he even discovered that many flowers, 

 such as Hemerocallis fulva, remained infertile if they were dusted 

 with their own pollen. 



Even the numerous experiments of that admirable German 

 botanist, C. F. Gartner, although they advanced matters further, 

 did not suffice to make the relations between insects and flowers 

 thoroughly clear; for this the basis of the theory of Descent and 

 Selection was necessary. Here, again, it was reserved for Charles 

 Darwin to lead the way where both contemporaries and predecessors 

 had been blindly groping. He recognized that, in general, self- 

 fertilization is disadvantageous to plants ; that they produce fewer 

 seeds, and that these produce feebler plants, than when they are cross- 

 fertilized ; that, therefore, those flowers which are arranged to secure 

 cross-fertilization have an advantage over those which are self- 

 fertilized. In many species, as Sprengel had already pointed out, 

 self-fertilization leads to actual infertility ; only a few plants are as 

 fertile with their own pollen as with that of another plant; and 

 Darwin believed that, in all flowering plants, crossing with others of 

 the same kind, at least from time to time, is necessary if they are not 

 to degenerate. 



Thus the advantage which the flowers derive from the visits of 

 insects lies in the fact that insects are instrumental in the cross- 

 fertilization of the flowers, and we can now understand how the plant 

 was able to vary in a manner favourable to the insect- visits, and to 

 exhibit adaptations which serve exclusively to make these visits easier; 

 we understand how it was possible that there should develop among 

 flowers an endless number of contrivances which served solely to 

 attract insects, and even how, for the same end, the insignificant 

 blossoms of the oldest Phanerogams must have been transformed into 

 real flowers. 



We must not imagine, however, that the obviously important 

 crossing of plant-individuals, usually called 'cross-pollination/ can 

 be effected only by means of insects. There were numerous plants 

 in earlier times, and there is still a whole series in which cross- 

 fertilization is effected through the air by the wind ; these are the 

 anemophilous or wind-pollinated Angiosperms. 



To these belong most of the catkin-bearers, such as hazel and 

 birch, and also the grasses and sedges, the hemp and the hop, and so 

 forth. In these plants there is no real flower, but only an incon- 

 spicuous blossom, without brightly-coloured outer envelopes, without 

 fragrance or nectar ; all of them have smooth pollen grains, which 



