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 Ferns retains the chief Algal characters, as 

 regards both its texture and its sexual process. The sporophyte, on the 

 other hand, 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 

 is typically sub-aerial in its characters, while the green Algae are typically 

 aquatic. A comparison of the successive families of the Archegoniate 

 series suggests the progress of the sporophyte, from small beginnings, as 

 illustrated in the Bryophytes, to larger size and greater complexity of form 

 and structure, as seen in the A T ascular Cryptogams and Gymnosperms : 

 its advance 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 reproduction 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 forms, perhaps thereby escaping from competition, spread to the 

 land, where access of water was only an occasional occurrence : in these 

 the sexual process could only be effected at time of rains or floods 

 or copious dews; and even then might not take place unless the sexual 

 organs were fully mature : thus less dependence could be placed upon 

 sexuality for propagation, and an alternative method of increase of individuals 



