Mutations 
itself the block does not return to its old position, 
but tips over and comes to rest on another facet, 
we have a representation of the kind of change 
indicated by a mutation. 
The analogy is far from perfect, for it makes 
it appear that the smallest mutation must of 
necessity involve a departure from the normal 
type more considerable than that of the largest 
fluctuating variation. Now, although mutations 
ordinarily consist in considerable deviations from 
the mean or mode of the type, while continuous 
variations are usually minute deviations, it some- 
times happens that the extreme fluctuations are 
more considerable than some mutations. Hence 
“fluctuating” describes this latter kind of 
variation more accurately than “continuous” 
does. 
The test, then, of a mutation is not so much 
the amount of deviation as the degree in which 
it is inherited. Mutations show no tendency to 
a gradual return to the mean of the parent 
species ; fluctuating variations do display such a 
tendency. A mutation consists, as M. E. East 
says, in the production of a new mode or centre 
for linear fluctuation ; it is, as it were, a shifting 
of the centre of gravity ; the centre about which 
those fluctuations which we call continuous varia- 
tions occur. 
As it is of considerable importance thoroughly 
to grasp the true nature of mutations or discon- 
79 
Laboratory of Ornithology 
159 Sapsucker Vigeds Roe 
Cartel University 
Utiaca, New York 
fassh 
