204 Proceedings of the Royal Physical Society. 



spores to escape these conditions. Dissemination of these 

 spores, for a plant living on the land, could only be obtained 

 by means of the wind, and a sporophore would become 

 essential for raising them as far as possible into the air. At 

 such times as the conditions became unfavourable, the result 

 of the conjugation would be to form this sporophore. From 

 the nature of the capsule of the moss plant, it is well seen 

 that variations must have occurred in this sporophore which 

 made it better able to resist the laud conditions than its 

 progenitor the gametophyte. The production of a trans- 

 fusion regulating apparatus, in the form of stomata, and a 

 true root which could penetrate the soil in search of water, 

 would give this sporophore advantages never attained to in 

 the gametophyte ; and gradually natural selection would 

 render it more and more perfect with powers of self- 

 sustenance and greater permanence, the formation of the 

 spores being put off to a later and later period. Having thus 

 more or less acquired the power of living in dry situations, a 

 new enemy in the form of its sister plants would come into 

 view, and its success in getting its food supply would depend 

 upon its power of overtopping its neighbours. The dangerous 

 existence of the gametophyte would be resorted to at longer 

 and longer intervals. The sporophyte acquiring great 

 specialisation in size, the possibilities of the spores germinat- 

 ing under conditions suitable for a successful issue from the 

 gametophyte would become more and more restricted. This 

 would inevitably be the case when through specialisation the 

 necessity arose for cross-fertilisation between the germs from 

 different gametophytes, and then further between those 

 produced by different sporophytes. The homosporous condi- 

 tion giving way to the heterosporous condition, the possibilities 

 of the germs meeting one another would become so remote 

 that any variation in the production of the spore, which 

 allowed it to become parasitic upon the asexual plant, would 

 be readily resorted to as the best method for the continua- 

 tion of the species. The possibilities of wide cross-fertilisa- 

 tion having thus been acquired by the system of pollenation, 

 the production of a seed having resistant properties and 

 capable of wide dispersal, would be the latest stage in the 



