GEOMETRICAL RATIO OF INCREASE 81 



some period of life. Our familiarity with the larger domes- 

 tic animals tends, I think, to mislead us : we see no great 

 destruction falling on them, but we do not keep in mind that 

 thousands are annually slaughtered for food, and that in a 

 state of nature an equal number would have somehow to be 

 disposed of. 



The only difference between organisms which annually pro- 

 duce eggs or seeds by the thousand, and those which produce 

 extremely few, is, that the slow-breeders would require a 

 few more years to people, under favourable conditions, a 

 whole district, let it be ever so large. The condor lays a 

 couple of eggs and the ostrich a score, and yet in the same 

 country the condor may be the more numerous of the two,- 

 the Fulmar petrel lays but one egg, yet it is believed to be 

 the most numerous bird in the world. One fly deposits hun- 

 dreds of eggs, and another, like the hippobosca, a single 

 one; but this difference does not determine how many indi- 

 viduals of the two species can be supported in a district. 

 A large number of eggs is of some importance to those spe- 

 cies which depend on a fluctuating amount of food, for it 

 allows them rapidly to increase in number. But the real im- 

 portance of a large number of eggs or seeds is to make up 

 for much destruction at some period of life; and this period 

 in the great majority of cases is an early one. If an animal 

 can in any way protect its own eggs or young, a small num- 

 ber may be produced, and yet the average stock be fully kept 

 up; but if many eggs or young are destroyed, many must be 

 produced, or the species will become extinct. It would suf- 

 fice to keep up the full number of a tree, which lived on an 

 average for a thousand years, if a single seed were produced 

 once in a thousand years, supposing that this seed were never 

 destroyed, and could be ensured to germinate in a fitting 

 place. So that, in all cases, the average number of any ani- 

 mal or plant depends only indirectly on the number of its 

 eggs or seeds. 



In looking at Nature, it is most necessary to keep the fore- 

 going considerations always in mind — never to forget that 

 every single organic being may be said to be striving to the 

 utmost to increase in numbers; that each lives by a struggle 

 at some period of its life; that heavy destruction inevitably 



