Definite and Indefinite Variability 



that variability is of two kinds definite and 

 indefinite. Indefinite variation is indiscriminate 

 variation in all directions around a mean, varia- 

 tion which obeys what we may perhaps call the 

 law of chance. Definite variation is variation in 

 a determinate direction variation chiefly on one 

 side of the mean. Darwin believed that these 

 determinate variations were caused by external 

 forces, and that they are inherited. He thus 

 accepted Lamarckian factors. " Each of the 

 endless variations," he writes, " which we see in 

 the plumage of our fowls, must have had some 

 efficient cause, and if the same causes were to 

 act uniformly during a long series of generations 

 on many individuals, all probably would be 

 modified in the same direction." 



But Darwin was always of opinion that this 

 definite variability, this variability in one direc- 

 tion as the result of some fixed cause, is far less 

 important, from an evolutionary point of view, 

 than indefinite variability, that it is the exception 

 rather than the rule, that the usual result of 

 changed conditions is to let loose a flood of 

 indefinite variability, that it is almost exclusively 

 upon this that natural selection acts. 



Darwin also recognised that variations differ 

 in degree, even as they do in kind. He per- 

 ceived that some variations are much more 

 pronounced than others. He recognised the 

 distinction between what are now known as 



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