382 Kauffman . — A Contribution to the 
in my cultures to the full extent of what Maurizio observed. Sometimes 
the hypogynous cell itself is transparent ; again, they may be filled with 
protoplasm ; the last condition is especially marked in calcium nitrate 
cultures. 
The comparisons just made serve to bring out clearly the possibility 
of inducing a variation which overlaps or imitates a character in a form 
which is apparently constant in nature ; and this fact naturally leads to 
a consideration as to the meaning and values of varieties and species, 
a theoretical question that may better be discussed in a separate section. 
The significance of the hypogynous cell has been fully discussed by 
Maurizio (loc. cit., p. 149), and my own results seem to bear out his conten- 
tion that the projection into the oogonium is not a fertilization tube, but 
represents merely a tendency in this genus to produce secondary growth, 
as in sporanges. It was often noticed that, even where a side-branch was 
fully developed and bore an antheridium of its own, these projections 
appeared to push up to a greater or less extent. Probably the hypogy- 
nous cell itself is no true antheridium, but merely an aborted or latent cell 
which has either lost its sexual function under ordinary conditions or had 
never attained it ; in either case we would hardly expect that its function 
could be so easily called forth as to form these projections as fertilizing 
tubes. Furthermore, it is not rarely absent altogether, and then the ordinary 
vegetative filament is the starting-point of the antheridial stalk. 
IV. 
There are several theoretical questions which this investigation touches 
and which may be briefly discussed. The first of these is the question of 
the constancy of species. As De Vries has pointed out, experimental 
evidence on this point is not very extensive. The expression has been 
used for a long time to indicate a constancy of characters during culture, 
viz. that if the individual remained constant in its specific characters it 
proved the autonomy of the species, while if it varied in a certain direction 
it belonged to another species, and the direction of variation indicated the 
species to which it belonged. Such cultures were usually made under so- 
called natural conditions. If, for example, a specimen of Saprolegnia was 
grown on a fly for three years and showed no change of a retrogressive or 
other nature in the characters to be tested, it was declared a definite species. 
If, on the other hand, the new characters disappeared at times, or were 
changed during the course of the cultivation to a character already known, 
the new form was considered merely a variety. 
This is the attitude at present exemplified by mycologists who de- 
scribe new species of the higher fungi. Often a third name is added to the 
binomial, and this implies that the new characters of the plant in question 
