i io Discussion on “ Alternation of Generations A 
form: but it should be carefully considered whether the 
9.15 possibilities of chromosome behaviour as seen now illustrated 
precisely what had happened in the past; whether the 
variations from the normal cycle alluded to were not ex post facto 
occurrences; they were not normal or even usual occurrences now, 
and it might be questioned whether they really repeated actual 
history. The normal cycle seemed to him the phenomenon with which 
we had mainly to do in all discussions on the origin of alternations ; 
if this were kept clearly in view tne historical distinctness of the 
two generations, even where their form was most alike, seemed 
naturally to follow. 
The President, Dr. D. H. Scott, said that he agreed very 
cordially with all that the last speaker had said in his general remarks 
on Dr. Lang’s theory. The first point he would like to make 
clear was that, in his opinion, Dr. Lang’s theory did not supersede 
the old views; it no more superseded the homologous theory than 
natural selection superseded evolution. So far from superseding 
the homologous theory the new hypothesis owed its point to the 
assumption that that theory was true. Dr, Lang’s theory was 
valuable just in so far as it helped to explain the great difference 
between the two generations, a difference which was the 
obvious difficulty when they were regarded as homologous. These 
differences were no difficulty on the antithetic or intercalation 
theory — the sporophyte being a new structure there was no reason 
why it should resemble the other and older generation. But when 
the two generations were regarded as being ultimately derived 
from similar individuals then the existing difference between them 
was the point to be explained ; Dr. Lang’s working hypothesis as 
to the origin of alternation . as it occurred in the higher plants, 
assumed, to use his own words, (New Phyt., Vol. VIII., p. 8,) that 
the archegoniate plants had arisen “ from forms in which a sexual 
(haploid) and asexual (diploid) generation of similar form alternated 
regularly.” This was precisely the homologous doctrine in so 
many words, as it had taken shape in their minds since the 
discovery of the cytological facts in Dictyota. These discoveries 
had shown that cytological differences did not preclude the two 
generations being homologous, and had thus completely removed 
the value of the cytological distinction as supporting the antithetic 
theory. For in the Dictyotacese we had two generations, one with 
the double, and the other with the single chromosome number, 
bearing spores and sexual cells respectively, and yet the two 
generations corresponded perfectly, as perfectly as Pringsheim 
could have desired. They owed a great debt to Pringsheim in this 
matter. He had laid stress on the almost absolute severance 
between the sexual and asexual individuals in certain Thallophyta 
and their consequent, more or less regular succession , 1 though no 
such good cases as had now been found in the Dictyotaceas and 
also in the Florideae were known to him. 
Dr. Lang’s leading idea was that the great difference between 
the two generations was to be explained by the enclosure of the egg 
in the maternal body, while the spore developed free. To sub¬ 
stantiate this idea it had to be shown that enclosure in the body of 
the parent actually produced a great effect in the right direction 
in analogous cases. Dr. Lang had mentioned the case of the 
1 Gesammelte Abhandlungen, Bd. II., p. 388. 
