D.—ZOOLOGY. 77 
confusion and interminable controversies about the inheritance of so- 
called ‘ acquired characters’ is due to the neglect of this important 
distinction. For it is quite clear that whereas factors may be trans- 
mitted, characters as such never are. The characters of the adult, 
being responses, are not present as such in the fertilised ovum from 
which it develops, they are produced anew at every generation.* No 
distinction in kind or value can be drawn between characters. 
lf some are inherited regularly and others are not, the distinction 
lies not in the nature or mode of production of the characters them- 
selves, but in the constancy of the factors and conditions which give 
rise to them. Thus, although there is only one kind of character, there 
are two kinds of variation. 
Much of the confusion in evolutionary literature is, I think, due 
to the use of the word variation in a loose manner. Sometimes it is 
taken to mean the degree of divergence between two individuals; some- 
times the character itself in which they differ, such as a colour or spot 
on a butterfly’s wing, at other times a variety or race differing from 
the normal form of the species. If clearness of thought and expression 
is to be attained, the word variation should mean the extent or degree 
of difference between two individuals or between an individual and the 
average of the species, the divergence of the new form from the old; 
not a new character or assemblage of characters, but a difference which 
can be measured or at least estimated. We shall then find that a 
yariation is of one of two kinds (which may, of course, be combined): 
the first kind is due to some change in the complex of effective 
environmental stimuli, the second to some change in the complex of 
_ germinal factors. 
- + 408 - 
The second kind, to which the name mutation has been applied, will, 
under constant conditions, be inherited since the new complex of 
factors will be transmitted to subsequent generations. The first kind 
of yariation, which has been called:a modification, will also be inherited, 
provided, of course, the change of stimulus persists. In either case, 
new characters will result. But here, again, we must be careful not to 
apply the terms mutation and modification to the characters themselves, 
as is so often done;* for we then reintroduce the confusion already 
exposed in the popular but misleading distinction between ‘ acquired ’ 
and ‘ non-acquired’ characters. The characters due to mutation or 
modification are, of course, indistinguishable by mere inspection, and 
can only be separated by experiment. A mutation once established 
should give rise, under uniform conditions, to a new heritable character, 
and may be detected by crossing with normal members of the species. 
3 In other words, all characters are ‘ acquired during the lifetime of the 
individual,’ and ‘inherited’ in the sense here defined has just the same 
meaning. Much the same view was advocated by Professor A. Sedgwick in his 
address to this Section at Dover in 1899, and it has also been developed by 
Dr. Archdall Reid and others. 
4 The name ‘mutation’ might be given to the alteration in the factors 
instead of the variation due to it. The latter might then be termed a muta- 
tional variation and would be opposed to a modificational variation At present 
the term ‘mutation’ is applied to three different things : the factorial change, 
? ‘ 
the variation or difference, and the new product response or character. 
