ORIGIN OF THE SPOROPHYTE 245 
The Archegoniatae themselves retain with remarkable pertinacity the 
awkward and embarrassing mode of fertilisation through the medium of 
external fluid water. But with the advent of the Seed-Habit this became 
modified: finally the sperm was no longer set-free as a cell motile in 
external water, but fertilisation came to be effected by means of a closed 
pollen-tube. Thus the higher Seed-Plants at last became typically terrestrial 
organisms, breaking away from the last vestige of the amphibious habit 
of their progenitors, the Archegoniatae. 
But all this was not achieved suddenly. From living organisms, and 
in some degree from fossils, indications may be gathered of the various 
steps which led to the establishment of the sporophyte as the essential 
eature Land- Tracing these steps backwards it is possible to 
obtain a clue from the simpler aquatic organisms: these plants give the 
best indication available how the initial start was probably made. There 
is reason to believe, on grounds of comparison, that the sexual generation 
or gametophyte was the prior existent, and that the neutral generation or 
‘sporophyte arose as a phase intercalated in the course of descent _between 
guccessive canes that the- total ae whieh—led—to—this_was—the- 
existence of tho licationsof-e ofWhich appear in so many 
of the lowerplants_as_a_consequence_of sexuality, and are connected 
with the reduction of chromosomes already doubled in the sexual fusion 
of nuclei. It is certainly the fact that in some Algae such post-sexual 
divisions do result in the production of a plurality of germs: biological 
circumstances which would encourage the multiplication of those germs 
might be expected to lead towards the establishment of a neutral generation. 
In plants exposed to changing conditions of moisture and of drought, 
such circumstances would be specially effective, and this must naturally 
be the position of any which spread to a land-habit. Here access to 
external fluid water wo €=an occasional rather than a constant 
occurrence : consequent! feould only be carried out occasionally, 
when water was available, ould be precluded under dry conditions. 
Less dependence could then be placed on sexuality for increase in 
number of individuals, and a _Dreminnt would be put on an alternative 
mode of propagation, suitable for dryer circumstances. The post-sexual 
divisions accompanying reduction would supply the initial state upon which 
variation and selection could work towards this end, and by an increase 
of these divisions the number of post-sexual germs would be increased. 
It is thus seen that the biological conditions involved in the transition 
from water to land would naturally encourage some form of amphibious 
alternation (Chapters V. and. VI.). 
Fhe-esteblishment_of a Land-Flora thus _involves-the-origin of a-body_ 
adapted _to terrestrial _life ; and as such the sporophyte is to be recog- 
nised. Its first function, as it is also its final office even in its most 
elaborate forms, is to produce spores. The spores of the simpler 
Archegoniatae are all similar and equivalent germs: the larger their 
