114 Evolution and Adaptation 
another in the struggle for existence, for, if true, it only 
goes to show more plainly how impossible it is to establish 
any safe scientific hypothesis, where the conditions are so 
complex and so impossible to estimate. To show that the 
young Scotch fir in an enclosed pasture is kept down by the 
browsing of the cattle, and in other parts of the world, Para- 
guay for instance, the number of cattle is determined by 
insects, and that the increase of these flies is probably habitu- 
ally checked by other insects, leads to a bewilderingly com- 
plex set of conditions. We cannot do better than to quote 
Darwin’s conclusion: “ Hence, if certain insectivorous birds 
were to decrease in Paraguay, the parasitic insects would 
probably increase; and this would lessen the number of the 
navel-frequenting flies—then cattle and horses would be- 
come feral, and this would certainly greatly alter (as indeed 
I have observed in parts of South America) the vegetation : 
this again would largely affect the insects; and this, as we 
have just seen in Staffordshire, the insectivorous birds, and 
so onwards in ever increasing circles of complexity. Not 
that under nature the relations will ever be as simple as this. 
Battle within battle must be continually recurring with vary- 
ing success ; and yet in the long run the forces are so nicely 
balanced, that the face of nature remains for long periods of 
time uniform, though assuredly the merest trifle would give 
the victory to one organic being over another. Nevertheless, 
so profound is our ignorance, and so high our. presumption, 
that we marvel when we hear of the extinction of an organic 
being ; and as we do not see the cause, we invoke cataclysms 
to desolate the world, or invent laws on the duration of the 
forms of life!” 
The effect of the struggle for existence in determining she 
distribution of species is well illustrated in the following 
cases : — 
“As the species of the same genus usually have, though 
by no means invariably, much similarity in habits and con-, 
