94 'WHAT IS A SPECIES?" 



the heritage of the whole ; by interbreeding excessive 

 spontaneous variation is checked, and the whole com- 

 munity of the species advances surely and with stability 

 into adjustment with the progressive changes of the 

 environment. 



We all remember Darwin's beautifully elaborated 

 metaphor 1 by which the past history of evolution 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 that Syngamy is represented by the 

 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 with its ever branching and beautiful 

 ramifications '. 2 



1 Origin of Species, 1859, p. 129. 2 loc. cit. p. 130. 



