SUMMARY. 197 



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

 dependently 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 inhabi- 

 tants 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 individuals and more diversified forms, and the com- 

 petition will have been severer, and thus the standard of 

 perfection will have been rendered higher. Natural selec- 

 tion 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 

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



It is generally acknowledged that all organic beings have 

 been formed on two great laws— Unity of Type, and the 

 Conditions of Existence. By unity of type is meant that 

 fundamental agreement in structure which we_ see in 

 organic beings of the same class, and which is quite inde- 

 pendent of their habits of life. On my theory, unity of 

 type is explained by unity of descent. The expression of 



