SUMMAR\. 185 



organs are closely examined, essential differences in their 

 structure can almost always be detected ; and this naturally 

 follows from the principle of natural selection. On the 

 other hand, the common rule throughout nature is infinite 

 diversity of structure for gaining the same end ; and this 

 again naturally follows from the same great principle. 



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 no* 

 have been slowly accumulated by means of natural selec- 

 tion. In many other cases, modifications are probably the 

 direct result of the laws of variation or of growth, indepen- 

 dently of any good having been thus gained. But even 

 such structures 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, believe that a part formerly of high importance 

 has frequently been retained (as the tail of an aquatic animal 

 by its terrestrial descendants), though it has become of such 

 small importance 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 success 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 

 Che 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 

 necessarily lead to absolute perfection ; nor, as far as we 

 can judge by our limited faculties, can absolute perfection 

 be everywhere 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 



