31 8 SIGNIFICANCE OF SEXUAL REPRODUCTION [V. 



After Darwin, several investigators, such as Kerner, Delpino 

 and Hildebrand, have paid further attention to the subject, but 

 it has been especially studied in a most thorough manner b}'' 

 Hermann Muller\ He looked at the subject from more than 

 one point of view, and showed by direct observation the species 

 of insects which effect cross-fertilization in various species of 

 our native flowers : he also studied the structure of insects in 

 relation to that of flowers, and attempted to establish the 

 mutual adaptations which exist between them. In this way 

 he succeeded in throwing much light upon the process of 

 transformation in many species of flowers, and in proving 

 that certain insects, although unconsciously, are, as it were, 

 breeders of certain forms of flowers. He not only distinguished 

 the disagreeably smelling, generally inconspicuous flowers 

 (' Ekelblumen ') produced by Diptera which live on putrid 

 substances, and the flowers which are produced by butterflies ; 

 but he also distinguished the flowers bred by saw-flies, by 

 Fossoria, and by bees. He even believes that in certain cases 

 [Viola calcaratd) he can prove that a flower which owed its 

 original form to being bred by bees, was afterwards adapted to 

 cross-fertilization by butterflies, when it had migrated into an 

 Alpine region where the latter insects are far more abundant 

 than the former. 



Although there must of course be much that is hypothetical 

 in the interpretations of the different parts of flowers offered 

 by Hermann Miiller, the majority of these explanations are 

 certainly correct, and it is of the greatest interest to be able to 

 recognise the adaptive characterof details, even when apparently 

 unimportant, in the structure and colours of flowers. 



Sachs has offered a very convincing explanation as to the 

 meaning of leaf-veining, and of its significance in relation to the 

 functions of leaves ^. He shows that the venation of a leaf is in 

 every case exactly adapted for the fulfilment of its purpose. 

 It has, in the first place, to conduct the nutrient fluid in both 



^ Compare Hermann Muller, ' Die Befruchtung der Blumen durch 

 Insekten und die gegenseitigen Anpassungen beider.' Leipzig, 1873. 

 See also many articles by the same author in ' Kosmos,' and other 

 periodicals. These later articles are included in the English translation 

 by D'Arcy W. Thompson. 



2 • Lectures on the Physiology of Plants,' translated by H. Marshall 

 Ward, Oxford, 1887, p. 47. 



