82 BIOLOGICAL ASPECT OF ALTERNATION 
to the air under comparatively dry circumstances, while dry weather is 
important for the dispersal of the spores, which it is the final function of 
the sporophyte to produce: thus the Fern, as we normally see. it, is an 
organism with, so to speak, one foot in the water, the other on the land. 
Calling in also such evidence from phylogeny as we can command, it 
will be generally accepted that the gametophyte is the older and pre- 
existent generation; it corresponds to the gametophyte as seen in the 
Liverworts, or in the green Algae: and if we trace the descent of the 
Archegoniate series from some green Algal forms, we may recognise 
that the gametophyte of the Hers retains the chief_A Eaeters, aS” 
i process. The sporophyte, on the 
and, is the younger generation: among the present green Algae 
there is hardly any body strictly comparable to the sporophyte, nor is. 
it to be expected that there should be, if, as above stated, the sporophyte 
Se while the green Algae are typically 
aquatic. .A_ comparison of the successive families of the Archegoniate 
eries suggests the te,-frem -small_beginnings,—as. - 
llustra@ ii the Bryophytes, to larger size. and. greater complexity of. form 
and structure, as seen in the Vascular Cryptogams and Gymnosperms: 
Wadvance is accompanied by a corresponding reduction of the gameto- 
phyte, and the whole is to be correlated with a progression from the 
aquatic or semi-aquatic habit of the lower forms, to the very distinctly 
sub-aerial habit of the higher. It may accordingly be concluded that the 
alternation which is{so prominent in the main Archegoniate series is the 
result of adaptation of originally aquatic organisms to sub-aerial conditions. 
of life: it may, in fact, be distinguished physiologically as an amphibious 
alternation, which finds its morphological expression in the difference of 
external form and internal,structure between the more ancient gametophyte 
and the more recent sporophyte. 
It is an important fact that in the main Archegoniate series the antithetic 
alternation is normally constant, though the balance of the two generations 
may vary: the constancy of the phenomenon makes us enquire why it 
should be so: the circumstances which have encouraged this constancy 
seem to have been these. The Archegoniate series probably sprang 
from green aquatic forms, inhabiting, as so many of the green Algae 
now do, shallow fresh water, or the higher levels between the marine 
tide-marks: the..sexual..reproduetion --was-~effected——through...the..means 
of external water.and if other conditions were favourable it could be 
carried out at any time through the water which was always present. 
Certain , perhaps thereby escaping from competition, spread to—the- 
“Tiere ses could. only be an_occasional occurrence: in _these_ 
the sexual proce ould. only..be_effected at time of rains or. floods. $s. 
“ok copious dewss and_even—then—might-not-take—place—unless_the sexual 
organs were fully mature: thus less dependence could be placed. upon 
sexuality for agation, and an alternative method of increase of individuals. 
