( cxvi ) 



are so evident in nature, that we are compelled to believe that 

 they meet and couuteract serious dangers which sooner or 

 later would menace the very existence of the species. And 

 among other adaptations it is significant that the instinct 

 under discussion should lead to the streaming of large popula- 

 tions, and not of small batches of individuals, from an area of 

 high-pressure." * 



It is impossible to consider the advantages which may 

 have favoured cross-fertilisation, if hereafter the generally 

 accepted physiological necessity turn out to be a delusion. 

 Brief reference may, however, be made to the special advant- 

 ages of community which are possible througli syngamy alone. 

 By inter-breeding the favourable variations arising in one 

 direction are combined with others arising in different direc- 

 tions ; by the kaleidoscopic changes produced by inter-breeding 

 more varied results are presented for selection, and the bene- 

 ficial qualities arising in one part of the mass may quickly 

 become the heritage of the whole ; by inter-breeding excessive 

 spontaneous variation is checked, and the whole community 

 of the species advances surely and with stability into adjust- 

 ment with the progressive changes of the environment. 



We all remember Darwin's beautifully elaborated metaphor t 

 by which the past history of evokxtion is shown forth in the 

 form and branching of a great tree. Darwin represented 

 species by the " green and budding twigs," and we may 

 suppose that the leaves stand for individuals, and tliat syn- 

 gamy is represented by tlie contact of leaf with leaf when the 

 branches sway in the wind. And just as contact may run 

 through large and small, irregular and compact masses of 

 leaves, so syngamy binds together groups of varying size and 

 distribution. So too a mass of foliage breached by a sudden 

 storm pictures for us the splitting of a syngamic chain into 

 two species by the disappearance of an intermediate link. 



It has been a pleasure to me that the central idea which I 

 have endeavoured to bring before you should be represented, 

 I trust without violence to the imagery, by means of "the 

 great Tree of Life, which fills with its dead and broken 

 branches the crust of the earth, and covers the surface Avith 

 its ever-branching and beautiful ramifications." | 

 * I. c. p. 464. t "Origin of Species, "< 1859, p. 129. J I. c. p. 130. 



