200 SUMMAEY. CiiAr. VI. 



portancc that it could not, in its present state, have been ac- 

 quired by natural selection — a power which acts solely through 

 tlic sur^-ival of tlie best-fitted individuals in the strufrgle for life. 



Natural selection will produce nothing in one sjiecies foi 

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

 produce parts, organs, and excretions, highly useful or even in- 

 dispensable, or highly injurious to another species, but in all 

 cases at the same time useful to the owner, Natural selection 

 in each well-stocked country must act chiefly through the com- 

 pc^tition of the inhabitants one with another, and consequently 

 will produce perfection, or strength in the battle for life, only 

 according to the standard of that countrj^ Hence the inhab- 

 itants of one countr}', generally the smaller one, will often yield 

 to the inhabitants of another and generally larger country. For 

 in the larger country there will have existed more individuals, 

 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 nccessarilj' pro- 

 duce absolute perfection ; nor, as far as Ave can judge by our 

 limited faculties, can absolute perfection be evei-ywhcrc found. 



On the theory of natural selection we can clearly inider- 

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

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

 the jiresent inhabitants of the world, is not strictly correct ; 

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

 not yet known, it must by my theory be strictly true. 



It is generally acknowledged that all organic beings have 

 been formed on two groat laws — Unity of Type, and the Con- 

 ditions of Existence. B}^ imity of type is meant that funda- 

 mental agTcement in structure which we see in organic beings 

 of the same class, and which is quite independent of their 

 halnts of life. . On my theory, unity of type is explained by 

 unity of descent. The expression of conditions of existence, so 

 often insisted on by the illustrious Cuvier, is fully embraced by 

 tlie principle of natural selection. For natural selection acts 

 by either now adapting the varying parts of each being to its 

 organic and inorganic conditions of life ; or by having adapted 

 i\unn during long-past periods of time : the adaptations being 

 aided in some cases by use and disuse, being affected by the 

 direct action of the external conditions of life, and being in all 

 crises subjected to the several laws of growth. Hence, in fact, 

 the law of the Conditions of Existence is the higher law ; as it 

 iiuhidcs, thrnugli the inheritance of former adaptations, that 

 of Unity of Type, 



