200 THE PHILOSOPHY OF BIOLOGY 



Therefore the mechanism and structure do actually 

 exist ! 



That which renders the son similar to the father — 

 the specific organisation — is undoubtedly very stable, 

 and it may persist in the face of a variable environment. 

 But now and then the son differs from the father. 

 The differences may be " accidental " and may not 

 be transmitted further — then we have to deal with an 

 unstable fluctuation ; or the differences may be 

 permanent — then we have to deal with a stable muta- 

 tion. What " produces " a mutation ? A change of 

 the environment, it may be said : if so, the mutation 

 is an active change or adaptation of the organism 

 to a change in its surroundings, and this adaptation 

 is a permanent one and is transmitted. Or the muta- 

 tion may be a spontaneous change of functioning. 

 If this disturbance of the stability of the organisation 

 is general, if it affects all the characters of the organism, 

 we have to deal with the establishment of a new 

 elementary species. But if the disturbance affects 

 only one, or a few characters, then we need not recognise 

 that a new elementary species has come into existence. 

 Men and women remain men and women (in their 

 morphology), although some time or other among the 

 brown eyes characteristic of a race blue eyes may have 

 appeared. The result of the disturbance, in this case, 

 has been to cause one, or a few, of the characters 

 that fluctuate to surpass their limits of stability. 



The idea of the elementary species is a clear and 

 simple one. It is a group of organisms connected by 

 ties of blood relationship : all have descended from 

 one pair of ancestors. The individuals exhibit certain 

 characters, all of which are variable. This variability 

 is not cumulative ; in generation after generation 

 the individuals of the species display variations which 



