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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