32 EVOLUTION OF LIVING ORGANISMS. 

 CHAPTER III 



DARWINISM AND HEREDITY 



SCARCELY more than half a century ago Darwin, in his 

 immortal work on The Origin of Species, first gave a 

 satisfying truly scientific explanation of the evolution of 

 living organisms. Before his time many authors had 

 recognised that the various forms of life have been 

 evolved from one another by gradual transformation. 

 From the earliest times, indeed, philosophers had specu- 

 lated on the possible modification of organisms, laying 

 stress now on external environment, now on internal 

 factors. All such speculations failed to convince, being 

 either obviously inadequate, or calling in mysterious 

 evolutionary forces of which no scientific explanation 

 could be given. Darwin first clearly showed how over- 

 whelming is the evidence that evolution has actually 

 taken place ; but his great merit is to have shown that 

 it can be accounted for by the action of " natural causes," 

 which can be seen at work at the present time, can be 

 tested by observation and experiment, and leave no room 

 for any mysterious governing causes in addition ; that, 

 in fact, a complete scientific aspect of the process of 

 evolution can be described as an unbroken series of 

 " natural " events, a sequence of cause and effect, a series 

 of steps each one strictly determined by that which came 

 before, and determining that which follows after. 



Darwin and Wallace simultaneously discovered the 

 great principle of natural selection, the keystone of the 

 Darwinian explanation of evolution. Like many great 

 truths, when stated it appears extraordinarily simple and 

 obvious. It can be described in the single phrase : the 

 survival of the fittest in the struggle for existence. If we 

 ask who are the fittest? the answer is those which 

 survive. This is no mere argument in a vicious circle ; 



