112 THE ORIGIN OF SPECIES 



fly, which lays its eggs in the navels of these animals 

 when first born. The increase of these flies, numerous 

 as they are, must be habitually checked by some means, 

 probably by other parasitic insects. Hence, if certain in- 

 sectivorous birds were to decrease in Paraguay, the para- 

 sitic insects would probably increase; and this would 

 lessen the number of the navel-frequenting flies — then 

 cattle and horses would become 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 onward 

 in ever-increasing circles of complexity. Not that under 

 nature the relations will ever be as simple as this. Bat- 

 tle 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 deso- 

 late the world, or invent laws on the duration of the 

 forms of life I % 



I am tempted to give one more instance showing how 

 plants and animals, remote in the scale of nature, are 

 bound together by a web of complex relations. I shall 

 hereafter have occasion to show that the exotic Lobelia 

 fulgens is never visited in my garden by insects, and 

 consequently, from its peculiar structure, never sets a 

 seed. Nearly all our orchidaceous plants absolutely re- 



I 



