CYTOLOGY OF PLANTS 309 
and here, just as in the case of plants, whole families 
may display this method of reproduction. 
We see, then, that the course of evolution in the 
vegetable kingdom would appear to have been accom- 
panied by a gradual increase in the 2x-generation 
at the expense of the x-generation. Starting with 
lowly aquatic organisms, and passing upwards through 
the mosses and ferns to the flowering plants, we find 
a steady diminution in the %-generation, whilst the 
vegetative labour of the plant is taken over by the 
ax-generation. It is, therefore, proper to suppose that 
organisms in which the main stage in the life-history 
is of double origin, and bears a double complement of 
hereditary factors, have some advantage over organisms 
in which this is not the case. We cannot, of course, be 
certain as to the exact nature of this advantage, but 
we may point out that it is only in the former kind 
of organisms that the operation of Mendel’s law can 
lead to the production of new combinations of parental 
characters in the body which represents the main stage 
of the life-history ; and that this circumstance may 
possibly lead to a greater power of adaptability to 
external circumstances. 
‘Perhaps the most interesting application of the infor- 
mation afforded by Mendel’s discovery is shown in its 
bearing upon the question of discontinuity in the origin 
of species. The fact of the definite and discontinuous 
inheritance of the differentiating features which dis- 
tinguish cultivated varieties from one another would 
point very plainly to a belief that such differences had 
arisen in a definite and discontinuous manner, even if 
