FACTORS OP EVOLUTION 3 



conditions of heredity, and see how the latest discoveries relating 

 to hereditary transmission are applicable in the field of social 

 evolution ; and we shall test the truth of Spencer's dictum that 

 there is no phenomenon in the world of social evolution that has 

 not its root in the conditions of organic life in general. 



Variability, heredity, excessive fecundity, and selection, are 

 the four factors which Darwin recognised as the basis of his 

 doctrine of descent. The co-operation of these four factors 

 results in the evolution of living organisms, their progressive 

 differentiation from a single original type and towards an ever 

 higher level. 



Variability is, in all probability, a property of every organism. 

 In the whole realm of nature there are no two living beings 

 exactly alike. We are, indeed, unable to penetrate into the 

 world of microscopic organisms deeply enough to appreciate 

 adequately the changes of structure or habit which occur there, 

 but from the examples afforded us in the world of macro- 

 organisms we are justified in concluding that Variability is the 

 universal rule. There is no valid reason for supposing that vari- 

 ability is absent in the simplest forms of life, or that its origin 

 is coincident with the development of higher forms. The micro- 

 organisms are subjected to the influence of their environment 

 as much as macro- organisms ; and no one at all aware of recent 

 researches can doubt that even diminutive creatures such as 

 bacteria are capable of varying greatly. 



The distinctive marks by which we recognise and differentiate 

 related species vary as a general rule within well-determined 

 limits. Where it is possible to obtain definite calculations we 

 usually find that fluctuating variations on either side of the 

 mean are by far the most numerous, and those variations which 

 break away from the normal type, either in a regressive or in an 

 ascendant direction, are less numerous. But this is not a 

 universal rule ; there are both symmetrical and asymmetrical 

 curves. Artificial breeding especially has disclosed cases like 



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