CYTOLOGY OF PLANTS 



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 2#-generation 

 at the expense of the ^-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 

 2#-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 



