CHAP, i The Nature of Alternation of Generations 59 



habitat and the difficulty of establishing itself must almost 

 necessarily be attended by a lessening of its powers of 

 obtaining nutriment, thus enforcing on the sporophyte the 

 development of vegetative powers, while the same necessity 

 was most likely to result in the disturbance of its sporo- 

 genous tissue, leading to partial sterilization. 



Bower summed up his views in 1890 as follows : ' The 

 archegoniate series is undoubtedly of Algal origin, and this 

 their gametophytes amply bear out : they probably sprang 

 from filamentous green aquatic forms, inhabiting shallow 

 fresh water or the higher levels between the marine tide- 

 marks. . . . Certain forms spread to the land where access 

 of water was only an occasional occurrence ; in these the 

 sexual progress 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 sexualty for propagation 

 and an alternative method of increase of individuals had 

 to be substituted. This was done by the production of the 

 sporophyte from the zygote ; once fertilized, a zygote might 

 in these plants divide up into a number of portions (carpo- 

 spores) each of which would then serve as a starting point 

 for a new individual, and dry circumstances under which 

 they would be powdery, would favour their dispersion. In 

 proportion as these plants spread to higher and drier levels 

 (in accordance with the advantage which they gained from 

 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 carpospores for pro- 

 pagation would increase ; consequently the number of 

 spores 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 external nourishment during their forma- 



