310 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 



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

 calcarat a) 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 character of details, even when apparently unimportant, 

 in the structure and colours of flowers. 



Sachs has offered a very convincing explanation as to the mean- 

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

 functions of leaves 1 . 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 direc- 

 tions, and in the second place to support the thin layers of assimi- 

 lating chlorophyll cells, and to stretch them out so as to expose as 

 large a surface as possible to the light ; lastly, it has to toughen 

 the leaf as a protection against being torn. He shows in a very 

 convincing manner that the whole diversity of leaf venation can be 

 understood from these three principles. Here, again, we meet with 

 purposeful arrangements in a class of structures in which it was 

 formerly thought that there was only a chaos of accidental forms, 

 or, as it were, the mere sport of nature with form. 



APPENDIX IV. ON THE SUPPOSED TRANSMISSION OF ACQUIRED 



CHARACTERS 2 . 



I 



"When I previously maintained that the proofs of the trans- 

 mission of artificially produced diseases are inconclusive, I had in 

 mind the only experiments which, as far as I am aware, can be 

 adduced in favour of the transmission of acquired characters ; viz. 

 the experiments of Brown-Sequard 3 on guinea-pigs. It is well 



1 ' Lectures on the Physiology of Plants,' translated by H. Marshall Ward, 

 Oxford, 1887, p. 47. 



2 Appendix to page 267. 



3 Brown-Sequard, 'Researches on epilepsy; its artificial production in animals 

 and its etiology, nature, and treatment.' Boston, 1857. Also various papers by the 

 same author in 'Journal de physiologic de 1'homme,' Tome I and III, 1858, 1860, 

 and in 'Archives de physiologic normale et pathologique,' Tome I-IV, 1868-1872. 



