AMPHIBIOUS HABIT 83 
had_to be substitutede Ihis_was done by the productionnofsthemsp 
phyte—from—the zygote: once fertilised a zygote. might..didexth pharrts= 
divide up into a number of~poerttens—(tarpospores), each of which would 
thén serve as a sta i fffdividual; and dry circumstances, 
under which they would be powdery, would favour their dispersion, as 
in the lower Liverworts. In proportion as these plants spread to higher 
and drier levels (in accordance with the advantage which they gained by 
escape from competition, and more free exposure to light for assimilation) 
the chance of a frequent recurrence of the circumstances necessary for 
sexual reproduction would be diminished, and the..dependence=-upon~ 
(StS O TES ano Rio pacation wou eens consequently the number of. 
pores produced by each sexually formed sporophyte must be larger, if 
the race is to survive, and be in a position to compete. Any increase 
in the number of spores entails greater supply of nourishment during 
their formation: this in the phylum of the Bryophytes is chiefly supplied 
from the gametophyte, which shows distinct adaptation to sub-aerial habit, 
while the means of nutrition on the part of the sporophyte itself are in 
these plants very limited, and the external morphological complexity of 
it very slight. In other phyla, however, such as the Filicales, Lycopodiales, 
and Equisetales, the sporophyte itself assumed the function of nutrition: 
a higher morphological differentiation of the parts followed, and a more 
clear distinction between the organs which. were to supply the nutriment 
(stem, leaves, and roots), and the parts devoted to the formation of 
spores (sporangia) : - for the first time stamped the sporophyte 
character _of independence and permanence, while the number of spores 
produced might’ now be practically unlimited: in these respects the 
Pteridophytes are immeasurably superior to the Bryophytes. ~Qne-strange 
point..in_the_whole_story..is,.howeyer,, the tenacity. with .which these.plamts~ 
(under the obvious disadvantages whiclf it entails when their habit is 
sub-aerial) retain their aquatic_type of fertilisation: it is only when we 
reach the Phanerogams, where the sporophyte attains its climax while 
the gametophyte is almost abortive, that we see the sexual process 
accommodated to that sub-aerial life which had led to the dominant 
position of the sporophyte; for in them the fertilisation is siphonogamic, 
being carried on by the pollen-tube: these plants are therefore independent 
of external fluid water for their fertilisation, and this fact has doubtless 
contributed largely to their present ascendency, When, as in the preceding 
sketch, we consider what the results of the migration from water to land 
must have been, the permanence and constancy of the antithetic alter- 
nation explains itself. The permanence or morphological fixity of a 
phenomenon in any phylum is in a sense proportional to its importance 
in the well-being of the organisms: given a conservatism in the mode of 
fertilisation (which it is difficult to explain), the rise and progress of 
the sporophyte in the Archegoniate series appears to be a_ natural 
outcome of the migration from water to land. 
