( 48 ) 
are so evident in nature, that we are compelled to believe that 
they meet and counteract 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 through 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. 
Weallremember Darwin’s beautifully elaborated metaphor T 
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 syn- 
gamy 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 ef 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.” { 
* le. p. 464. + ‘Origin of Species,” 1859, p. 129. 1 2.1, Derlads 
