412 NOTES OF A BOTANIST CHAP. XXIV 
This is a great advance on the views stated 
in the earlier letter, in which he wrote : " The ants 
cannot be said to be useful to the plants, any more 
than fleas and lice are to animals ; and the plants 
have to accommodate to their parasites as best they 
may." The evidence, however, now shows that, in 
all probability, they are always useful, in which case 
their becoming hereditary is merely a question of 
variability in the plant, and the continued preser- 
vation of those whose variations were in the direc- 
tion of utility to the ants. 
The whole of these very interesting phenomena, 
so well described by Spruce, are thus seen to be in 
complete accordance with those of the modification 
of flowers by insect-agency, which are now admitted 
to depend upon a mutual adaptation for the benefit 
of both plant and insect. 
They lead, I think, to the establishment of the 
general principle, that no special adaptation of one 
organism to another can become fixed and hereditary 
unless it is of direct utility to both.] 
