62 GEOMETraCAL RATIO OF INCREASE. 



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 

 produce eggs or seeds by the thousand, and those which 

 produce extremely few, is, that the slow breeders would re- 

 quire a few more years to people, under favorable condi- 

 tions, 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 Qgg, yet it is be- 

 lieved to be the most numerous bird in the world. One 

 fly deposits hundreds of eggs, and another, like the hippo- 

 bosca, a single one. But this difference does not determine 

 how many individuals of the two species can be supported 

 in a district. A large number of eggs is of some im- 

 portance to those species which depend on a fluctuating 

 amount of food, for it allows them rapidly to increase in 

 number. But the real importance 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 number 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 suffice 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 insured to germinate in a fitting 

 place; so that, in all cases, the average number of any an- 

 imal or plant depends only indirectly on the number of its 

 eggs or seeds. 



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

 foregoing 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 falls either on the young or old during each gen- 

 eration or at recurrent intervals. Lighten any check, mit- 

 igate the destruction ever so little, and the number of the 

 species will almost instantaneously increase to any amount 



