THE FACTORS OF EVOLUTION 173 



only quite recently that we have arrived at a just and critical 

 appreciation of the theory of selection, and naturalists, while 

 recognizing its strong points, point out to-day its weaknesses to 

 an increasing degree. 



The theory of natural selection is the doctrine of chance. 

 According to it the individual possesses no formative power in 

 the development of its body. By chance it receives at birth, 

 through an accidental variation in one or another part of its 

 organism, some useful character which the other innividuals of its 

 species do not receive. But however useful this new acquisition 

 may be to the animal, it is not able to develop it. Only by 

 pairing with another individual which by chance possesses the 

 same useful modification, may this new character be improved 

 in the next generation. The only participation of the living 

 body in the development of its useful characters lies, therefore, 

 in reproduction. Of the causes which lead to the origin of such 

 variations this theory does not supply any explanation. It takes 

 variability as granted ; and since chance is the determining 

 factor it is obvious that there will arise equally numerous cases 

 of useful, indifferent and harmful variations. We can therefore 

 already see that selection, in order to be at all effective, must 

 work with an immense number of variations which form the 

 material from which the struggle for existence makes its selection 

 on the ground of usefulness. 



That variability is universal in Nature is well known, but the 

 necessarily large number is said to be due to the excess of births, 

 because each species produces far more descendants than will 

 reach maturity. If we consider that many fishes annually pro- 

 duce hundreds of thousands of young, and that parasitic worms 

 produce millions of germs, this demand made by natural selection 

 seems justified, for with such a wealth of descendants it is perfectly 

 intelligible that chance would be able to produce a sufficient 

 number of variations to give rise to all the useful characters 

 found in the organisms. But we also know animals for instance, 

 the elephant, and, in fact, most mammals, as well as numerous 

 species of birds which possess a very low ratio of increase. Nor 

 must it be forgotten that in the animals with an immense progeny 

 most germs are destroyed as ova or immature individuals, and 

 thus the material for the stage at which the fiercest competition 



