78 



SCIENTIFIC NEWS. 



[July 27, I J 



within, say 150 yards. But this place, situated in the 

 midst of the plain, seems exposed (at least in the upper 

 layers of the superincumbent atmosphere) to an enormous 

 current of air from the north-east, coming through a pass 

 in the mountain chain of the Chartreuse, opening upon 

 the valley of the Isere at an altitude of about 2,600 feet 

 above the latter. 



I cannot too strongly insist upon the absolute agree- 

 ment of M. Glbert's observation with all mine. The 

 aspect of the furrow traced through or in, not over the 

 dusty soil is scrupulously accurate. This furrow records 

 in a manner the oscillations of the base of the whirl- 

 wind in its twofold movement of translation and rotation. 



(To be continued.) 



THE STABILITY OF THE FAUNA. 



IS the proportion of the several animal species in any 

 country fluctuating or permanent ? In spite of the 

 occurrence of such well-known facts as the decline and 

 ultimate disappearance of certain living forms within 

 historical ages, Professor K. Fuchs, of Pressburg 

 (Humboldt) , argues with great ability that, disregarding 

 trifling changes, these proportions may remain unaltered 

 for thousands of years, if only the climate and the 

 vegetation do not vary. He, of course, excepts the direct 

 or indirect agency of man, which makes itself widely 

 felt as a disturbing factor, and that very frequently to 

 his own injury. The Professor shows that the animal 

 world is self-regulating : any increase or decrease in the 

 numbers of some given species involving at once a 

 corresponding-— and sometimes more than proportionate 

 — decrease or increase in some one or more other 

 species. This fact, apparently very simple, involves a 

 number of reciprocal bearings not unworthy of a passing 

 notice. 



We may first glance at the mean duration of the life of 

 animals. It may be readily shown that the mean life of 

 an animal is inversely as its reproductive power, i.e., the 

 more fertile any species the shorter is the life of the 

 individual, whilst animals which multiply slowly 

 necessarily reach a high age. Let us assume, to prove 

 this point, that the reproduction of the hare is 2 per cent, 

 or one-fiftieth daily, that is, among 100 hares two are 

 born daily (a quite arbitrary figure). In this case the 

 number of hares can be permanent only on condition 

 that their daily mortality is also 2 per cent, or one- 

 fiftieth out of every 100 hares, two dying on the average 

 daily. In other words, the death-rate must balance the 

 birth-rate. Great fertility is, therefore, for the indi- 

 vidual a mournful gift of Nature. We find, in fact, that 

 small creatures which multiply very rapidly, are 

 correspondingly short-lived. The small mean duration 

 of life is generally effected amongst animals by a 

 numerous destruction of the eggs and the young brood, 

 and it does not, theoretically speaking, prevent indi- 

 viduals from attaining a high age. 



What is the practical value for an animal of endurance, 

 the power of prolonging its life under unfavourable 

 circumstances, such as poor food, deficient shelter, a bad 

 climate, in short, conditions which make existence a 

 burden ? Animals which, like most beasts of prey, 

 rarely meet with a violent end will generally increase 

 until, by dint of deficient food, etc., the death-rate equals 

 the birth-rate. In enduring animals, especially if they 

 have the misfortune to increase rapidly, so that great 

 mortality is needed to effect an equilibrium, as among 



the dog tribe, this condition is reached only when the 

 general want has become very urgent. They are thus, 

 so to speak, predestinated to distress, and their charac- 

 teristic attributes, such as outrageous voracity, restless 

 prowling and searching, have been evolved in conse- 

 quence. Unenduring animals, i.e., such as quickly 

 perish under unfavourable circumstances, and which, in 

 addition, increase slowly, so that a balance is reached 

 even with a low death-rate, are destined for a lordly 

 existence. As soon as want sets in their mortality 

 rises, their numbers fall, and single individuals obtain 

 scope, in consequence of decreasing competition, to find 

 better food and shelter. 



Endurance, combined with rapid increase, presents 

 advantages where times of scarcity and times of plenty 

 alternate, say in the succession of summer and winter, 

 or of dry and wet seasons. The numbers which such 

 species may reach at the end of the good season render 

 it probable that few places of shelter will remain un- 

 discovered and unutilised, whilst the power of endurance 

 will minimise the number of individuals which perish in 

 the bad season. Hence we may expect to find enduring 

 and fruitful animals in extreme climates, and non-endur- 

 ing and slowly-multiplying species in equable climates. 



We shall find much instruction from the theoretical 

 case of a country inhabited only by a single food-animal, 

 say the hare, which can find food without limit, and 

 which encounters only one devourer, say the fox. It 

 will be seen at once that in such a case the foxes, faring 

 abundantly upon the plentiful hares, will increase, and 

 in virtue of their growing numbers will extirpate the 

 hares daily more and more until the number of the latter 

 is so far reduced that the foxes cannot catch a sufficiency, 

 and begin to die out from want until they can catch no 

 more hares than are born in the meantime. We then 

 find an equilibrium. The hares can be considered as 

 rendered stable when- .so many foxes are present as to 

 devour the daily increase of the hares, sc that the 

 number of hares remains unaltered. On the other hand 

 the number of foxes may be called stable when hares 

 have become so few that, in spite of all exertion, as many 

 foxes die from famine as are born in the meantime. 



As soon as any species increases above the equilibrium, 

 it digs its own grave : the fox, in the case we have been 

 supposing, by eradicating the hares, and the hares by 

 facilitating the increase of the foxes. Inversely each 

 species improves its condition by falling below the 

 normal number : the fox, by rendering an increase of 

 the hares possible, and the hares by contributing to the 

 dying out of the foxes. 



A very interesting and seemingly paradoxical result 

 is that as both the devourer and the devoured reach higher 

 numbers the more difficult it becomes for the robber to 

 seize his prey, the harder the booty is to catch. In pro- 

 portion as the conditions of the chase become less easy, 

 the numbers of the food-animal increase, since the 

 devourer meets with his minimum supply only when the 

 food-animal is numerous. But — and this is the peculiar 

 point — the devourers will increase because, from the great 

 numbers of their prey, more of the former can exist 

 than previously. Nature, therefore would do the beasts 

 of prey an ill service if she made the chase easier, either 

 by giving them greater resources, or by placing at their 

 disposal more helpless and unprotected food-animals. 

 They would soon devour down their prey to such a low 

 number that few devourers could exist. 

 (To be continued.) 



