EVOLUTION, VARIATION AND HEREDITY 463 



was well wooded. They destroyed all the forests by eating up the 

 seedlings as they appeared. With the forest disappeared also the 

 birds and insects living in them, insect-fertilised flowers were also 

 affected, and, indeed, even the climate was changed. 



The argument set out in the Origin of Species may be 

 briefly summarised by what is known as Wallace's chart : 



Fact. Consequence. 



A. Rapid increase in numbers } 



B. Total number of species V . Straggle for Existence. 



stationary ) 



C. Struggle for Existence ? Survival of the Fittest 



D. Variation with Heredity f h 



E. Survival of the Fittest ) Structural modification and differ- 



F. Change of Environment \ ' * entiation of species. 



Darwin's own conclusions on the matter are best summarised 

 in his own words. " I have now recapitulated the facts and con- 

 siderations which have thoroughly convinced me that species have 

 been modified, during a long course of descent. This has been 

 effected chiefly through the natural selection of numerous slight, 

 favourable variations ; aided in an important manner, that is, in 

 relation to adaptive structures, whether past or present, by the 

 direct action of external conditions, and by variations which seem 

 to us in our ignorance to arise spontaneously. It appears that I 

 formerly underrated the frequency and value of these latter forms 

 of variations, as leading to permanent modifications of structure 

 independently of natural selection." 



Here then we have a statement of the case as Darwin conceived 

 it, and in it we have the phenomena of evolution definitely envisaged 

 and set forth. This was done so successfully that it has never been 

 seriously doubted since, and herein lies Darwin's great service to 

 Biology. He put forward an enormous amount of evidence demon- 

 strating evolution in such a cogent manner that even the most 

 sceptical were convinced. With it he also proposed a theory to 

 account for the causes responsible for evolution, which is quite 

 logical and conceivable. The Biologist, however, is not entirely 

 concerned with logical possibilities, and he desires to know if this is 

 actually the way in which evolution was brought about* Herein, 

 as pointed out previously, subsequent workers have not always 

 agreed with Darwin. 



The publication of the " Origin of Species " exerted a profound 

 influence upon biological work, and marked a complete change in the 

 scientific outlook, 



