596 Species According to the Theory of Mutation. 
These causes may fall into two entirely different cat- 
egories ; on the one hand, they may consist in insufficient 
systematic knowledge, on the other, in the insufficient num- 
ber of experimental crosses. With regard to the former 
point it should be remembered that, although the sys- 
tematist frequently takes latent characters into considera- 
tion, it is obviously by no means always possible to de- 
cide on systematic grounds whether a character which 
we do not see is really absent or exists only in a latent 
condition. Nevertheless, latency is often regarded as a 
retrogressive metamorphosis and therefore as the mark 
of a variety; whereas absence is considered as a phylo- 
genetically older step and therefore as a specific character 
(see above p. 71). 
From this discussion we see that we may cross a plant 
in which any given character is active either with one in 
which the internal factor for this character is absent, or 
with a species or variety in which it is present but in a 
latent or inactive state. Externally there is no difference 
w* 
between two such crosses, but fundamentally they are 
exactly opposite, and therefore it is to be expected that 
their results will differ. The cross, active X absent is a 
uni-sexual union and will presumably lead to a halving 
of the external characters of the parents in the hybrid, 
whereas the cross active X latent is a bi-sexual one, and 
follows MENDEL' s laws, at least in ordinary cases. Many 
paradoxes, which at present seem to negative the parallel 
between systematic and sexual affinity may perhaps be 
explained by more exact investigation on these lines. 
FOCKE gives the following cases as instances: 1 "Silenc 
vidgaris, and 5*. maritima, Capsella rubella and C. bursa 
pastoris, Phaseolus vidgaris and Ph. niultiflorus, or the 
1 FOCKE, loc. at., p. 448. 
