172 BOTANICAL GAZETTE [SEPTEMBER 
races has become grown over, even a relatively strong reaction 
may not be evident. The growth of only a few hours may render 
a regrading of a culture impossible. 
In the foregoing pages some of the sources of experimental 
error likely to be encountered in studying the sexual reactions 
between races of mucors have been pointed out, and the technique 
which a familiarity with these sources has led the writers to adopt 
has been outlined. What has been written may serve as an introduc- 
tion to a paper to follow in this journal, in which it may be possible 
to show that the discordant conclusions reached by certain inves- 
tigators working with the same material may have been due to 
differences in the methods employed. 
STATION FOR EXPERIMENTAL EVOLUTION 
CoLtp Sprinc HarBor, 
