Mutations and Evolution. 
223 
on the facts of apogamy and apospory in Ferns, and experimentally 
induced apospory in Bryophytes (Lang, 1901). But even when of 
natural occurrence, these are generally admitted to be exceptional 
conditions of recent development. The careful cytological studies 
of apogamy and apospory, however, led Farmer and Digby (1907) to 
the conclusion that these phenomena leave the question ofalternation 
essentially w'here it was. It need only be pointed out here that the 
theory of homologous alternation of generations in plants corresponds 
to such a view as would restrict the significance of recapitulation to a 
great degree; while an important phase of the antithetic theory is 
that it implies a lengthening of the sporophyte generation in 
connection with the adaptation of plants to a terrestrial habitat. 
In his original studies of apogamy and apospory, Lang (1898) 
carefully refrained from giving his views a bias in either direction. 
More recently (Lang, 1909) he has propounded an interesting con¬ 
ception of homologous alternation from an ontogenetic point of 
view. 1 Briefly the view begins with the concept of the species cell, 
based on the fact that any cell of the species is potentially able to 
reproduce the whole plant. This being the case, “the haploid and 
diploid germ-cells have potentially the same morphological 
properties.” But they are believed to develop different generations 
because the germ cell is exposed to different conditions in initiating 
the two generations. This interesting suggestion, however, 
encounters many serious difficulties, some of which were pointed 
out in a criticism by V. H. Blackman (1909). 
‘ It cannot be said that the homologous view has received wide 
support as regards Archegoniates and their descendants, but on the 
other hand important evidence has developed for the occurrence of 
homologous alternation in Algae. It is significant that in this group 
the gametophyte and sporophyte generations develop under the 
same relatively uniform conditions, exposed to sea water, and they 
are morphologically alike. Without discussing the subject in any 
detail, 2 it may be pointed out that Yamanouchi (1906) found in the 
red alga Polysiphonia an alternation of generations, which he 
regarded as antithetic, between tetrasporic plants having 40 chromo¬ 
somes and sexual plants with 20, the cystocarp being a part of the 
sporophytic phase. Lewis (1909), in a study of another of the 
Rhodophycese, Griffithsia, has taken up a more advanced position 
1 See also the discussion on alternation of generations at the Linnean 
Society, published in New Phytol. 8 : 104-116, 1909. 
a For recent discussions of the life-cycles in Algae see Fritsch (1916) and 
Davis (1916). 
