1899] THE SCOPE OF NATURAL SELECTION 121 



even if the actual number of inhabitants remain more or less the same 

 as when the struggle first commenced. The selection of variations 

 will thus tend to pass through certain more or less ill-defined but 

 nevertheless real stages. In proportion as the struggle becomes in- 

 tense, either from the number or from the increasing adaptability of 

 the organisms, or both, certain major essential adaptations, which were 

 necessary for the climatic and other more or less comparatively simple 

 conditions, will be supplemented by minor auxiliary variations which 

 in the earlier stages would not have appeared. And still later as 

 more and more rigorous conditions of life were imposed the advantage 

 would tend to rest with those organisms which possessed highly co- 

 ordinated adaptations, since this would entail more rapid responsiveness 

 to environment. 



As evolution advances from the unspecialised to the specialised, 

 and higher and higher forms of life come into being, with increasing 

 complexity and specialisation of parts entailing an increasingly deli- 

 cate adjustment of those parts to each other's needs, the relation of 

 each part to the whole organism becomes of more and more import- 

 ance, and it follows that selection must become more and more gener- 

 alised in its action. No single variation could be of service to any of 

 the higher forms of life unless it was in more or less complete harmony 

 with the whole tendency of the individual. The adjustment of parts 

 and their mutual interdependence make it essential for adaptation 

 that the relation of parts be preserved ; consequently, correlated 

 minute favourable variations will tend to be more and more selected 

 as evolution passes from the unspecialised to the specialised forms of 

 life. This response of the whole organism should be still more deli- 

 cate in those forms of life that are continually subjecting themselves 

 to changed conditions ; hence this delicacy of adjustment is far more 

 necessary in the higher forms of animal life than in the more stationary 

 plant organisms, and in the developing nervous system of animals 

 we have just the central adjusting system that is required for these 

 conditions. With evolution of type, there will thus he an increasingly 

 definite tendency given to organic, especially the animal, forms of life, 

 if the acting principle of evolution has been selectional. Selection 

 is therefore able to account for the steadily progressive tendency of 

 life as a whole without calling to its aid any unknown and doubtful 

 perfecting principle. 



To summarise : — Natural selection, acting on the whole organism, 

 tends to produce more and more definite tendencies in all surviving 

 forms of life, which tendencies are progressive and continuous in 

 character. Variable conditions, by partially altering the line of 

 selection, induce a temporary indefiniteness. And lastly, the process 

 of selection being itself able to be the indirect, though not the direct, 

 cause of those favourable variations, which it subsequently selects from, 

 is able to dispense with any subsidiary factors, provided it has a 



