Notes. 
587 
form is this : Is the sporophyte in the Bryophytes and the Vascular 
Cryptogams to be ultimately traced back to modification of a genea- 
logical individual homologous with the gametophyte, or is it the 
result of still further elaboration of an interpolated stage more or 
less like that seen in Coleochaetel On the antithetic theory the 
sporophyte is traced increasing in complexity through a series of 
forms illustrated by Oedogonium , Coleochaete , Riccia, Marchantia , 
An/hoceros, and the simplest sporophytes of the Vascular Cryptogams 
are regarded as having been derived from a sporogonium, which 
already possessed a considerable amount of sterile tissue. If, on 
the other hand, we apply the homologous theory, several alternatives 
present themselves. The first, which is not widely different from 
the antithetic theory, is that in the course of its descent the sporo- 
phyte of the Vascular Cryptogams has passed through a stage 
resembling the Bryophyte sporogonium, but that the origin of this 
second generation in the ancestral Algae was homologous. But the 
homologous theory does not necessarily assume the existence of the 
sporogonial stage. The sporophyte of the Vascular Cryptogams 
may have had an independent origin from that of the Bryophyta, 
and have resulted from the modification of individuals, which were 
never reduced to the condition of a fruit body. 
As to the circumstances which led to alternation of generations, the 
two theories are in essential agreement. We owe to Professor Bower 
the general statement, which must serve as the starting-point of any 
explanation, that the origin of the alternation may be correlated with 
a change of habit from aquatic to sub-aerial life. This holds whether 
the second generation is considered to be homologous with the first, 
or to be the result of interpolation. On the latter view, which is that 
elaborated by Professor Bower, the importance of the drier conditions 
of life is sought in the prevention of repeated acts of fertilization. It 
would thus have been an advantage to the organism to produce many 
individuals as the result of one sexual act, and this is seen to be 
effected with increasing perfection as we pass from the simpler to 
the more complex Bryophyte sporogonia, and from these to the 
Pteridophyta. The same change of environment may, however, have 
initiated the modification of individuals, which were originally potential 
sexual plants, into spore-bearing forms. We shall return to this when 
discussing apogamy. 
We have seen that the facts of morphology do not of themselves 
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