574 Species Accord iug to the Theory of Mutation. 



same leaf cannot be at once trifoliate and quinquefoliate 

 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 varialjility 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 l)etween two t}pes 

 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 

 usuallv so described, but this is only true in the sen.se 

 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 

 solelv 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 fe. g.. 

 Fig. 27 of the first volume, page 138). In the case of 

 tricotyls and syncotyls the half race rarely contains more 

 than a verv 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- 



