Alternation of Generations based on Ontogeny. 9 
association of the two generations may thus have been indepen¬ 
dent in the various groups of the Bryophyta and Pteridophyta. 
This is a matter for enquiry in each case and may be regarded as 
a quite open question. The modification of the attached genera¬ 
tion may thus have been from the outset on different lines in the 
Bryophyta and Pteridophyta, or in the groups of the latter, such 
as the Lycopodiales and Filicales. A polyphyletic origin of the 
groups of archegoniate plants would probably involve great differ¬ 
ences in the sporophytes from the outset, partly because of the 
differences of the retained specific cells, but also because the sub¬ 
stratum formed by the sexual generation which influences the 
retained cell would be different in each case. 
The germ-cells of the diploid generation, i.e. the spores, are 
assumed to have continued to be shed freely from the plant, though 
now provided with walls (if these were wanting in the ancestral 
forms) and adapted to aerial dispersal. They would serve as the 
means of spreading the organism to a distance. 
On this hypothesis, the two generations in each species are 
regarded as homologous, in that they correspond to regularly 
succeeding individuals (sexual and asexual), developed from germ- 
cells which are similar in their morphogenetic powers. The degree 
of homology which can actually be traced in the vegetative structure 
of the individuals of the two generations is a matter for special 
enquiry in each case. It may range from practically complete 
identity as in Dictyota, to cases in which the development of the 
dependent generation is from the first so different to that of the 
free-living one, that the tracing of homologies between the bodies 
of the two forms is out of the question. Extreme examples of the 
latter case, such as the sporogonium of Riccia, have formed important 
links in the arguments for an antithetic origin of the second 
generation. 
As regards the number of chromosomes in the nuclei and 
the mode of reproduction, the two generations are assumed to have 
been different in the ancestral forms. The asexual reproductive 
organs may, however, have been homologous with the sexual organs 
in these. Homologies, may therefore be looked for between the 
reproductive organs of the two generations of the same plant in the 
Bryophyta and Pteridophyta. 
The further evolution of the organisms with definite alternation 
of unlike generations would involve the occurrence of variations and 
their selection. Without entering into the question fully, it should 
be pointed out that any variation must be assumed to affect the 
