8 
W. H. Lang. 
of the growing young sporogonium. A similar view may perhaps 
be taken of the ovule and seed. 
We may now proceed to apply the point of view to which we 
have been led from a consideration of the relation of the two 
generations in the ontogeny to the phylogenetic problem of the 
origin of the two generations as'we find them in the Bryophyta and 
Pteridophyta. 
The question of the origin of the alternation of similar genera¬ 
tions as found in Dictyota and Polysiphonia may be dismissed as 
not falling within the scope of the present enquiry. Recent inves¬ 
tigations have shown that important light may be expected on this 
question as our detailed knowledge of the types of alternation in 
the various groups of Algae increases. Without suggesting that any 
particular form among the existing Algae was an ancestor of the 
archegoniate plants, the origin of the latter may, in the light of our 
present knowledge, fairly be assumed to have been from forms in 
which a sexual (haploid) and asexual (diploid) generation of similar 
form alternated regularly. The origin of the two generations in 
Archegoniate Plants need not now be sought in a differentiation of 
forms bearing, according to the external conditions, sexual or 
asexual reproductive organs on the same individual. 
The development of the individual from the fertilised egg 
within the body of the parent individual would result from the egg 
remaining in the organ in which it was formed and being sought 
out and fertilised there by the motile spermatozoid. The association 
of the fertilised egg with the maternal body would give the pos¬ 
sibility of the latter influencing the developing zygote physiolo¬ 
gically. The change from the extruded to the detained ovum may 
well have coincided with the departure from a purely aquatic mode 
of life. At least it would have suited this departure. The relation 
between the spread of organisms to the land and the origin of 
alternation as we see it in the archegoniate plants was first clearly 
pointed out by Bower. It applies equally to a homologous theory, 
but it should be noted that the relation suggested here is rather a 
result of the terrestrial life than an adaptation to it. Once the 
dependent relation of the diploid generation was established, pro¬ 
found and probably sudden changes might be expected to follow, 
resulting in the difference in body form between sporophyte and 
gametophyte. 
It is probable that the spread to land would lead to the 
association of the two generations in more than one set of organ¬ 
isms, whether at the same or at different geological periods. The 
