io6 Discussion on “ Alternation of Generations 
proceeding from the enclosing individual. The importance of these 
influences can hardly be exaggerated. It is in relation to these 
differences in the conditions of development of the spore and zygote, 
and so far as we know always in relation to differences of this kind, 
that the marked differences in the alternating individuals appear. 
The case of Polysiphonia is peculiarly interesting. We here find 
(i) the haploid tetraspore, developing free and giving rise to a plant; 
(ii) the diploid cell resulting from fertilisation, developing in relation 
to the tissues of the sexual plant into a system of sporogenous 
filaments; (iii) the diploid carpospore, developing free and giving 
rise to an individual like that which came from the haploid tetra¬ 
spore. The view that the haploid and diploid cells have similar 
morphogenetic properties is thus supported by the same facts which 
are difficulties in the way of the explanation first considered. Further 
it simplifies the question of the transmission of characters in the 
organism and allows of one set of determinants serving for both 
generations. 
I n the plants we are considering, the actual mode of reproduction, 
though not the formation of the reproductive organs, appears to be 
intimately and perhaps necessarily associated with the cytological 
difference between the two generations. 
The view to which we have been led, that the development of 
the zygote in relation to the enclosing individual is the important 
factor leading to the different organisation of the sporophyte as 
compared with the gametophyte in the same life-history, is an 
ontogenetic view. It is readily applied, however, to the phylogenetic 
problem of the origin of alternation as we see it in Bryophyta and 
Pteridophyta, the typical land plants, in which the marked differences 
of the two alternating generations are found. For the non-extrusion 
or retention of the egg would fit well with the departure of the 
organism from aquatic life, and the non-mobile egg and not the 
spermatozoid or the spore would be the likely structure to be 
retained. The retained generation would thus naturally happen to 
be the diploid one. As a matter of fact we find the retention of the 
egg characteristic of the Archegoniatse as contrasted with most 
Algae. In this sense I can adopt Professor Bower’s important cor¬ 
relation of the origin of alternation, as we see it in archegoniate 
plants, with the spread to land. 
If this ontogenetic view is correct we should be justified in 
seeking for correspondences in the vegetative organs, and possibly 
also in the reproductive organs, between the two individuals of the 
same life-cycle. These correspondences—though between haploid 
and diploid individuals—I should term homologies, since they may 
amount to practical identity when the conditions of development 
are exactly the same. We should expect to find correspondences 
most recognisable where the conditions of development are partly 
similar, and perhaps wanting when the conditions are most 
completely different. 
I shall now pass in brief review some well known facts in 
illustration of the hypothesis I have advanced. The particular 
applications of this, which I shall venture to make, are of course 
far more tentative than the view of the importance of the development 
of one generation enclosed in the other, as leading to the difference 
between them. The facts will be useful also in recalling what any 
sufficient hypothesis must cover, 
