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 

 feature of a Land-Flora. 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 

 successive gametophytes : that the initial step which led to this was the 

 existence of those complications of cell-division which appear in so many 

 of the lower plants 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 would be an occasional rather than a constant 

 occurrence : consequently sexuality could only be carried out occasionally, 

 when water was available, while it would be precluded under dry conditions. 

 Less dependence could then be placed on sexuality for increase in 

 number of individuals, and a premium 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.). 



The establishment 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 



