82 BIOMETRY 
is wanting, that new species have arisen exclusively 
through the accumulation by natural selection of 
variations of a strictly indefinite, fluctuating, or normal 
kind. We have already seen reasons for believing that 
this is very far from being the case, and future chapters 
will be found to add considerably to the force and 
quantity of the evidence already adduced. 
Normal variations, strictly speaking, are individual 
differences which can be supposed to depend upon a 
large number of small factors or causes—factors so 
numerous and so minute that the numerical distribu- 
tion of the individuals examined, when ranged in order 
according to the feature chosen for examination, is 
found to conform closely to that which would be 
expected on the mathematical theory of chance. 
Such a distribution will only result when the differences 
considered can be strictly regarded as lying upon a 
linear scale, and when they are also evenly distributed 
along that scale. That is to say, the biometrician 
deals with continuous variations of a quantitative kind. 
It is to be hoped that these somewhat obscure sayings 
will be more easily understood in the light of what 
follows. 
The facts of variation have not been found readily 
amenable to precise definition, but we shall endeavour 
to make plain by the aid of a few examples what kinds 
of variations do and what kinds do not appear to be 
legitimate objects for the application of biometrical 
methods. Thus it may be thought that the biome- 
trician is outrunning his license when he ranks the 
colours shown by the iris of the human eye in a con- 
