168 Experimental Zoology 
Cases of blended inheritance especially seem to come under 
this heading. But so long as we do not know definitely what 
occurs in these cases, it seems to me arbitrary to speak of unit 
characters as immutable and quite unnecessary to make this idea 
a cardinal point of the mutation theory. The behavior of cer- 
tain characters in heredity shows that they do act as units, and it 
is a great convenience to deal with them as such, but unnecessary 
to push the matter so far as to hold that they are immutable. 
If unit characters can be halved, altered, added to, or changed in 
any way, their immutability does not seem to be an essential 
point of their characterization, and, as has been said, there is 
some evidence to indicate that such changes may take place. 
The idea of immutability is not likely to suggest itself when results 
of this kind are considered, but in cases where complete domi- 
nance of one form over the other occurs, the idea of unit char- 
acters is more likely to arise. If, however, as J think probable, 
we are dealing here with alternate or contrasted characters that 
cannot both develop at the same time, the isolation of the char- 
acters in question is due to their mutual exclusion rather than to 
their immutability. Even if it be true that mutations take place 
by definite steps without the presence of intermediate forms, it 
does not necessarily follow that these steps may not subsequently 
become subdivided by each crossing with the parent form. The 
central idea of mutation remains, even if the unit-character that 
marks the steps is capable of being changed. 
The results of hybridizing of forms differing by a single unit- 
character seem to show that when in the first generation the 
hybrids (F;) are strictly like one of the two parents, the hybrids 
of the new generation (fF) also show complete development! of 
one of the two characters in the extracted dominants and of the 
other in the extracted recessives. This may be called the law of 
incompatibility. If, on the other hand, the dominance is incom- 
plete, z.e. if both dominate in the first generation, there is in- 
complete dominance in the second generation. This may be 
called the law of compatibility. If further facts establish these 
? In the sense of contrast, not necessarily of actual separation. 
