386 The Outline of Science 



When we are concerned with making a good lawn we may 

 pursue two methods. We may eliminate the weeds or we may 

 foster by suitable tonics the growth of the grass. Similarly, in 

 Nature's sifting there is lethal selection, which works by eliminat- 

 ing the relatively less fit to given conditions of life, and there is 

 reproductive selection, which works through the predominant in- 

 crease of the more successful. Darwin never thought simply of 

 Natural Selection ; he always emphasised its manifold and subtle 

 modes of operation. He saw, for instance, what some of his sue 1 

 cessors missed, that the sifting need not in the least involve a 

 sudden cutting off of the relatively less fit, for a shortened life and 

 a less successful family will in the long run bring about the same 

 result as a drastic pruning. It should not be necessary to point 

 out that "the survival of the fittest" does not necessarily mean 

 the survival of the strongest or cleverest or best ; it simply means 

 "fittest" relatively to particular conditions. The tapeworm is 

 a fit survivor as well as the Golden Eagle. 



Darwin realised what some of his successors have missed, 

 that even slight peculiarities may be of critical moment when 

 tested in relation to the complex web of life in which the creature 

 has its being. This is very important in regard to the general 

 progressiveness of evolution that new departures are sifted in 

 reference to a slowly wrought out and firmly established system 

 of inter-relations. (See the article on Inter-relations.) 



10 

 Sexual Selection 



Many male animals, such as stags, antelopes, sea-lions, black- 

 cock, and spiders, fight with one another at the mating time, 

 competing for the possession of females. According to Darwin, 

 "the strongest and, with some species, the best-armed of the 

 males drive away the weaker; and the former would then unite 

 with the more vigorous and better-nourished females, because 

 they are the first to breed. Such vigorous pairs would surely 



