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and self-sown firs ai'e now springing up in multitudes, so close together that 

 all cannot live. When I ascertained that these young trees had not been sown 

 or planted, I was so much surprised at their numbers that I went to several 

 points of view, whence I could examine hundreds of acres of the unenclosed 

 heath, and literally I could not see a single Scotch fir, except the old planted 

 clumps. But on looking closely between the stems of the heath, I found a 

 multitude of seedlings and little trees, which had been perpetually browsed down 

 by cattle. In one square yard, at a point some hundred yards distant from 

 one of the old clumps, I counted thirty-two little trees ; and one of them, with 

 twenty-six rings of growth, had during many years tried to raise its head, and 

 had failed. No wonder that, as soon as the land was enclosed, it became 

 thickly clothed with vigorously growing young firs. Yet the heath was so ex- 

 tremely barren and so extensive, that no one would ever have imagined that 

 cattle would have so closely and effectually searched it for food. 



" Here we see that cattle absolutely determined the existence of the 

 Scotch fir in this particular locality and under the conditions indicated ; 

 but in several parts of the world insects determine the existence of cattle. 

 Perhaps Paraguay offers the most curious instance of this ; for here neither 

 horses, nor cattle, nor dogs, have ever run wild, though they swarm northward 

 and southward in a feral state ; and Azara and Rengger have shown that this 

 is caused by the great number in Paraguay of a certain 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 

 birds. Hence, if certain insectivorous birds (whose numbers are probably 

 regulated by hawks or beasts of prey) were to increase in Paraguay, the flies 

 would decrease — 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 onwards in ever 

 increasing circles of complexity. We began this series by insectivorous birds, 

 and we have ended with them. Not that in nature the relations can ever be 

 as simple as this. Battle within battle must ever be recurring with varying 

 success ; and yet in the long run the forces are so nicely balanced, that the face 

 of nature remains uniform for long periods of time, though assuredly the 

 merest trifle would often 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 same author further adds : — ■ 



" I am tempted to give one more instance showing how plants and animals, 

 most 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, in this part of England, is never visited by insects, and consequently, 

 from its peculiar structure, never can set a seed. Many of our orchidaceous plants 

 absolutely require the visit of moths to remove their pollen -masses, and thus to 

 fertilize them. I have, also, reason to believe that humble-bees are indispen- 

 sable to the fertilization of the heartsease (Viola tricolor), for other bees do not 

 visit this flower. From experiments which I have lately tried, I have found 

 that the visits of bees are necessary for the fertilization of some kinds of clover ; 

 but humble-bees alone visit the red clover (Trifolium pratense), as other bees 

 cannot reach the nectar. Hence I have very little doubt, that if the whole 

 genus of humble-bees became extinct or very rare in England, the heartsease 

 and red clover would become very rare, or wholly disappear. The number of 

 humble-bees in any district depends in a great degree on the number of field 

 mice, which destroy their combs and nests ; and Mr. H. Newman, who has 



