IN THE THEORY OF NATURAL SELECTION. 281 



to a leaf had made its first appearance. Conversely, this latter 

 resemblance could not develope before the butterfly had gained the 

 habit of holding- its wings in the appropriate position. Both 

 characters must therefore have come into existence simultaneously, 

 and must have undergone increase side by side : the marking 

 progressing from an imperfect to a very close similarity, while 

 the position of the wings gradually approached the attitude 

 which was exactly appropriate. The development of certain 

 minute structural elements of the central nervous system, and the 

 appropriate distribution of colouring matter on the wings, must 

 have taken place simultaneously, and only those individuals have 

 been selected to continue the species which possessed the favourable 

 variations in both these directions. 



It is, however, obvious that sexual reproduction will readily 

 afford such combinations of required characters, for by its means 

 the most diverse features are continually united in the same indi- 

 vidual, and this seems to me to be one of its most important 

 results. 



I do not know what meaning can be attributed to sexual repro- 

 duction other than the creation of hereditary individual characters 

 to form the material upon which natural selection may work. 

 Sexual reproduction is so universal in all classes of multicellular 

 organisms, and nature deviates so rarely from it, that it must 

 necessarily be of pre-eminent importance. If it be true that 

 new species are produced by processes of selection, it follows that 

 the development of the whole organic world depends on these pro- 

 cesses, and the part that amphigony has to play in nature, by 

 rendering selection possible among multicellular organisms, is not 

 only important, but of the very highest imaginable importance. 



But when I maintain that the meaning of sexual reproduction is 

 to render possible the transformation of the higher organisms by 

 means of natural selection, such a statement is not equivalent to 

 the assertion that sexual reproduction originally came into exist- 

 ence in order to achieve this end. The effects which are now pro- 

 duced by sexual reproduction did not constitute the causes which 

 led to its first appearance. Sexual reproduction came into existence 

 before it could lead to hereditary individual variability. Its first 

 appearance must therefore have had some other cause ; but the 

 nature of this cause can hardly be determined with any degree of 



