CHAP. T The Nature of Alternation of Generations 63 



prove that the sexual and asexual methods of reproduction 

 can be brought about at will by varying the conditions 

 under which the plants are kept. It follows necessarily that 

 the forms are all homologous. This, at first sight, seemed 

 to be strong evidence against the antithetic theory, but 

 reflection shows that it is not necessarily so. The advocates 

 of the theory are not and have not been unanimous in 

 trying to include the Thallophyta in their scheme at all, 

 Bower, especially, holding that any regular alternation of 

 sexual and asexual forms in that group indicate parallel 

 development rather than stages in phylogenetic sequence. 

 Nor can the varying behaviour of the nuclei in the Thallo- 

 phyta be held more pertinent to the inquiry, on account of 

 the plastic nature of the whole group. 



No further discovery of importance fell within our 

 period. The controversy which was being conducted with 

 so much warmth extended over the remaining years of the 

 century, both theories attracting supporters, who, how- 

 ever, did little if anything more than re-state the old argu- 

 ments, based upon the long-known facts. With the dawn 

 of the new century, further research led to the elucidation 

 of certain points bearing on the theories, and opening the 

 way to their further discussion from slightly different points 

 of view. The year 1900, then, saw several different views 

 in favour. One school found antithetic alternation regular 

 and constant throughout the archegoniate plants, but not 

 represented with any certainty among the Thallophyta. 

 Another held that its inception can be traced there in 

 such forms as Oedogonium and Coleochaete, and among the 

 Florideae and the Ascomycetes. Yet a third claimed that 

 all the forms are homologous, and that the differentiation in 

 the Archegoniatae is but the evolution of a differentiation 

 commencing in and perhaps extending throughout the lower 

 group. All agreed that in the Thallophyta there exists 

 an alternation of an irregular and intermittent character, of 



