ALTERNATION OF GENERATIONS. 
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6. The problem which is met by the theory of alternation of generations in the form 
in which it has now been presented, is to refer the most important stages in the 
history of development of all plants to a single scheme which is illustrated most clearly 
in the cases of Muscineae and Filices, where Hofmeister first discovered this alternation 
in 185 1. The same botanist was also the first to explain the development of the seed 
in Gymnosperms by the alternation of generations in Lycopodiaceae, and hence to 
compare it with the same phenomenon in Filices and Muscineae. At the present time 
our knowledge of the development of Thallophytes has made so much progress that 
it is possible to determine what are its main features, and to compare them with, those 
of Muscineae and vascular plants. This comparison, which has only been briefly 
indicated above, will be followed out more in detail in the sequel, and will lead to 
the result that Thallophytes may also be included in the scheme under which the 
other classes are comprised, the first stage of development closing with the formation 
of sexual organs, from which proceeds the second generation, essentially different 
from the first, and closing with the production of true spores. It will therefore show 
that the development of all plants which possess sexual organs may be divided into 
two stages which correspond in all essential points to the two generations in the life- 
history of a Fern; and that there is, therefore, in the whole vegetable kingdom, 
only one type of alternation of generations so far as it is brought about by sexual 
organs. 
