3 82 



SCIENTIFIC NEWS. 



[Oct. 12, I? 



ON THE STABILITY OF THE FAUNA 

 OF ANY COUNTRY. 



(Continued from p. 78.) 



WHAT circumstances render it possible for a food- 

 species to become very numerous ? Above all, 

 as it has been already intimated, great protection. It 

 seems paradoxical to say that the more voracious the 

 devourers — i.e., the larger the minimum quantity upon 

 which they can prolong life, the more numerous becomes 

 the devoured species. A greedy devourer can exist only 

 where great masses of nutriment are available. He 

 begins to die out as soon as the food-species declines in 

 number, and thus the increase of the latter is again 

 rendered practicable. It would be worst for the mice if 

 shrews could increase everywhere freely, and on the 

 the other hand the mice would fare best, and would 

 multiply most rapidly if the lion had to support himself 

 upon mice. Great fecundity affords to a food-species no 

 advantage, their numbers being determined only by the 

 dexterity of the devourer, and the minimum ration upon 

 which he can subsist. 



What circumstances allow the devourer to become 

 most numerous ? The first condition, as we have already 

 seen, is that he must find hunting as difficult as possible, 

 as he otherwise would reduce too greatly the numbers 

 of the animal upon which he preys ; the second condi- 

 tion is high fecundity of such species, so that, if we 

 may use the expression, the capital of food-animals may 

 bring in a high interest. The fecundity of the victim, 

 therefore is an advantage not to him, but to the de- 

 vourer. 



Circumstances take a different aspect when we have 

 to consider not two, but several animals. 



Such beasts of prey as feed upon the same species 

 may be called competing devourers, which unite their 

 powers to root out the victims. Those devourers which 

 hold out longest, i.e., which by reason of their skill in 

 the chase, and their power of enduring hunger can still 

 maintain themselves on a very scanty supply of victims 

 will come off victorious. The less skilful and more 

 greedy species perish of want. Whilst, therefore, a 

 beast of prey existing without rivals, finds its advantage 

 in small skill and great need of food ; this advantage, in 

 case of competition becomes a grave disadvantage. 

 Nature encounters here a dilemma which she solves 

 very ably. She is continually creating new devourers and 

 equipping them with greater skill in pursuit, so that 

 they may find it possible to rout their old competitors. 

 But now, lest the world should become constantly 

 poorer in individuals, she creates at the same time 

 victims which are more and more skilful in concealment, 

 whereby the success of all devourers is equalized, and 

 that of the new beasts of prey is of course depressed, 

 and the number of individuals is increased. We have 

 thus a race among the devourers which is never ended 

 because the goal, the victim, escapes just as rapidly. 



Those food-animals which are decimated by the same 

 enemies may be called fellow-victims. Those which are 

 hardest to catch will multiply most rapidly, and by their 

 great fecundity will support such an army of devourers 

 that all their slower and less fruitful fellow-victims will 

 be rooted out, unless secured by especially favourable 

 circumstances. 



The most fruitful and the best concealed species thus 

 eliminate their fellow-victims by the instrumentality of 

 beasts of prey. As fellow-victims are for the most part 



at the same time competitors {i.e., as the quail and the 

 partridge, both victims of the fox, are competitors on 

 the corn-field) fecundity and concealment benefit a 

 victim in as far as they hinge on the suppression of 

 competitors by rearing a great army of devourers. 



What circumstances determine the number of a 

 species ? Such number is stable when mortality and 

 fecundity balance each other. Fecundity is heightened 

 by abundant normal food and general comfort, and de- 

 creased by every kind of privation. But what deter- 

 mines mortality ? The existence of every animal 

 species depends on a number of conditions (temperature, 

 food-plants, dryness, nesting-places, etc.), and where any 

 one of these conditions is in default the species cannot 

 exist. If, e.g., we were to draw up a map of Europe 

 for the capercailzie (Tetrao urogallus), marking the places 

 of suitable temperature ; on a second map the spots 

 supplying suitable food ; on a third, a third condition. 

 We should then find that all the marked places would 

 coincide only at a few spots. For the urogallus, Europe 

 is not a continent, but a group of some large islands. 

 For the viper, Europe might thus be called an archipe- 

 lago of very small islands. Probably many an animal 

 which we regard as rare has reached in its world the 

 highest possible number, but this world, in which alone 

 it is able to live, is only a small isolated part of a con- 

 tinent or a sea. From such centres, the species range 

 on all sides as visitors, but without any one forming a 

 permanent settlement. 



Within the habitable districts, the existence and the 

 plenty of a species depends especially on its relation to 

 other species, the question being whether the local 

 conditions expose the animal to its enemies, or afford it 

 the opportunity of concealment. We have seen that the 

 greatest abundance of animals will occur where their 

 pursuit is the most difficult. Hence we can under- 

 stand that certain animals, not dangerous to each other, 

 and dependent on the same conditions, do not co-exist in 

 the same regions or countries, but seem to avoid each 

 other. If two victims have the same enemies, but in 

 district A, Nature favours the concealment of one, but 

 in B, that of another, then in each district the more pro- 

 tected animal will indirectly extirpate the less protected, 

 by increasing rapidly, and thus enabling the devourers 

 to multiply to such an extent that they destroy the other 

 victim. If, conversely, of two devourers which depend 

 on the same diet, here the one and there the other is 

 favoured in the chase by the nature of the ground, then 

 in each case the stock of food animals is depressed to 

 such an extent that the competitor is starved out. 



A beast of prey is formidably armed if he can feed 

 upon more species than his rivals. Suppose the de- 

 vourer, A, lives exclusively upon .mice, and requires a 

 stock of 1,000 per square mile, whilst the devourer, B, 

 lives exclusively upon small birds, and likewise requires 

 1,000 per square mile. If now a fresh devourer, C, makes 

 his appearance in the district, able to live both on mice 

 and on birds, and needing only 1,000 food-animals per 

 square mile, he then reduces the total stock of both mice 

 and birds to 1,000, and thus starves out A and B. 



The worst inmates of a district are a beast of prey 

 which devours very many species, and a food-animal 

 which is very fruitful and conceals itself well. As an 

 instance of the latter kind we may take the mouse, but 

 for which we should have a greater variety of small 

 animals. The fauna of any district is the richer the 

 more specialised is the nutriment of the beasts of prey. 



