SUMMARY 217 



In many cases we are far too ignorant to be enabled to 

 assert that a part or organ is so unimportant for the welfare 

 of a species, that modifications in its structure could not 

 have been slowly accumulated by means of natural selection. 

 In many other cases, modifications are probably the direct 

 result of the laws of variation or of growth, independently 

 of any good having been thus gained. But even such struc- 

 tures have often, as we may feel assured, been subsequently 

 taken advantage of, and still further modified, for the good 

 of species under new conditions of life. We may, also, be- 

 lieve that a part formerly of high importance has frequently 

 been retained (as the tail of an aquatic animal by its terres- 

 trial descendants), though it has become of such small im- 

 portance that it could not, in its present state, have been 

 acquired by means of natural selection. 



Natural selection can produce nothing in one species for 

 the exclusive good or injury of another ; though it may well 

 produce parts, organs, and excretions highly useful or even 

 indispensable, or again highly injurious to another species, 

 but in all cases at the same time useful to the possessor. In 

 each well-stocked country natural selection acts through the 

 competition of the inhabitants, and consequently leads to suc- 

 cess in the battle for life, only in accordance with the 

 standard of that particular country. Hence the inhabitants 

 of one country, generally the smaller one, often yield to the 

 inhabitants of another and generally the larger country. 

 For in the larger country there will have existed more indi- 

 viduals and more diversified forms, and the competition will 

 have been severer, and thus the standard of perfection will 

 have been rendered higher. Natural selection will not neces- 

 sarily lead to absolute perfection ; nor, as far as we can judge 

 by our limited faculties, can absolute perfection be every- 

 where predicated. 



On the theory of natural selection we can clearly under- 

 stand the full meaning of that old canon in natural history, 

 "Natura non facit saltum." This canon, if we look to the 

 present inhabitants alone of the world, is not strictly cor- 

 rect; but if we include all those of past times, whether known 

 or unknown, it must on this theory be strictly true. 



It is generally acknowledged that all organic beings have 



