H4 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 the 

 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- 



