Robertson: Theories of Evolution. 247 
minute variations of an unfavourable kind tended to the ex- 
tinction of the creatures exhibiting them. The type of 
variability expressed in these minute individual differences is 
called ‘normal’ or ‘fluctuating’ variability, and is characterised 
by the fact that it conforms to the law of chance. As an 
instance, we may take some statistics given by Galton of the 
strength of pull of 519 men between 23 and 26 years of age. 
Strength of pull. Number of cases. 
Under 50 lbs. 2 
” 60 ” 8% 
5 70 5 27 % 
” 80)" 55 aga 
” gO x)’ 21 % 
», 100 ,, 4% 
Over 100 ,, Bie 
We notice from this table that the largest class have a pull 
of 70-80 lbs; that is to say the moderately strong individuals 
are more numerous than are the extremely strong or extremely 
weak, and we also notice that the numbers decrease gradually 
as we pass towards either extreme. In other words, ‘normal’ 
variability behaves just as we should expect if it conformed to 
the laws of probability. 
In speaking of Darwin’s. work I have mentioned the 
difficulty of the ‘uselessness of minimal variations;’ an 
adaptation in its fully devoloped form may be very useful to 
the creature possessing it, and yet it is often difficult to see how 
its rudimentary beginning could be useful enough to be pre- 
served if it were due to the gradual accumulation of the minute 
changes due to fluctuating variability. This difficulty, along 
with other considerations as to the nature and inheritance of 
normal variations which I cannot enter upon here, have led 
some biologists to the view that species have not arisen by 
fluctuating variations, but have progressed by jumps, separated 
by periods of stability ; such jumps have been named ‘mutations.’ 
In discussing the possibility of mutations the analogy has been 
suggested of a polygon which is ina position of stable equilibrium 
when it is standing on one face. When it is tipped up it is not 
in a position of equilibrium at all, and either falls back on to the 
original face or forward on to the next face. In the second case 
it has, as it were, gone through a mutation and reached a second 
position of stability. 
A number of cases have been reported in which a ‘mutation’ 
seems to have been completely stable. The Ancona sheep 
1907 July 1. 
