5/4 Species According to the Theory of Mutation. 
same leaf cannot be at once trifoliate and quinque foliate 
and so on ; in a word, an organ cannot be both normal and 
abnormal. 
These vicarious pairs of characters are the sources 
of a great variability inasmuch as the anomaly can ap- 
pear in all degrees of development. In such cases the 
individuals of a group are not ranged round a mean in 
respect of their external qualities, as with ordinary fluc- 
tuating or oscillating variability, but between two types 
which are often widely separated and more or less anti- 
thetic to one another. They have the appearance of being 
inconstant; and races and varieties of this kind are 
usually so described, but this is only true in the sense 
that the range of forms which they present is a very 
wide one; and, moreover, is ditypic or dimorphic. But 
it would not be true in the sense that any individual could 
transgress the boundaries of this range and found a new 
race. In this sense the so-called inconstant races are just 
as stable as the best constant species and varieties. 
The difference between half and middle races lies 
solely in the difference between the mutual relation of the 
members of the vicarious pair in the two cases. If, 
under ordinary conditions, and in the absence of selection, 
one of them predominates over the other to a very large 
extent, the race is, so to speak, unilateral and is called a 
half race (e. g., Fig. 135). But if, under similar cir- 
cumstances, neither of them predominates but an equilib- 
rium is maintained, we have an intermediate race (e. g.. 
Fig. 27 of the first volume, page 138). In the case of 
tricotyls and syncotyls the half race rarely contains more 
than a very few anomalous individuals, in the absence 
of selection ; whilst the intermediate race consists as a 
rule about half of normal, and the other half of tricotvl- 
