v.] IN THE THEORY OF NATURAL SELECTION. 319 



directions, and in the second place to support the thin layers of 

 assimilating 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 \ 



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'^ on guinea-pigs. It is 

 well known that he produced artificial epilepsy in these ani- 

 mals by dividing certain parts of the central and also the 

 peripheral nervous system. The descendants of the animals 

 which acquired epilepsy sometimes inherited the disease of 

 their parents. 



These experiments have been since repeated by Obcr- 

 steiner^, who has described them in a very exact and entirely 

 unprejudiced manner. The fact itself cannot be doubted : it 

 is certain that some of the descendants of animals in which 

 epilepsy has been artificially produced, have also themselves 

 suffered from epilepsy in consequence of the disease of their 

 parents. This fact may be accepted as proved, but in my 

 opinion we have no right to conclude from it that acquired 

 characters can be transmitted. Epilepsy is not a morphological 

 character; it is a disease. We could only speak of the trans- 

 mission of a morphological character, if a certain morphological 



1 Appendix to page 274. 



2 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 

 I'homme,' Tome I and III, 1858, i860, and in ' Archives de physiologic 

 normale et pathologique,' Tome I-IV, 1868-1872. 



2 'Oesterreichische medicinische Jahrbuchcr.' Jahrgang, 1875, p. 179. 



