356 The American Naturalist. [May, 



but variable equilibrium between these two functional kinds 

 of organs; and especially in the transformation of sporophyl- 

 lary organs to vegetative ones. If these disturbances, espe- 

 cially in the nature of partial or complete loss of carbon 

 a ssi m i Kaing organs of the sporophyte produce such an effect, 

 why should there not be a similar influence brought to bear 

 on the sporophyte, when the same function resides solely in 

 the gametophyte, and a disturbing element of this kind is 

 introduced? To me there are convincing grounds for believ- 

 ing that this influence was a very potent, though not the only 

 one in the early evolution of sporophytic assimilatory organs. 

 By this I do not mean that in the Bryophyta, for example, in- 

 jury to the gametophyte would now produce distinct vegeta- 

 tive organs on the sporophyte, which would tend to make it 

 independent of the gametophyte. But that in the Bryophyte- 

 like ancestors of the Pteridophytes an influence of this kind 

 did actually take place appears to me reasonable. 



In the gradual passage from an aquatic life, for which the 

 gametophyte was better suited, to a terrestrial existence for 

 for which it was unadapted, a disturbance of this function was 

 introduced. This would not only assist in the sterilization of 

 some of the sporogenous tissue, which was taking place, but 

 there would also be a tendency to force this function upon 

 some of the sterilized portions of the sporophyte, and to expand 

 them into organs better adapted to this office. As eruptions 

 in the mass of sporogenous tissue took place and sporophylls 

 were evolved, this would be accompanied by the transference 

 of the assimilatory function of the gametophyte to some of these 

 sporophylls. Even the protophylls may have originated by 

 the eruption of certain of the sterile portions of the sporophyte 

 under the influence of disturbed nutrition. 



The sporophyte from its nature presented greater possibil- 

 ities in the way of the elaboration of a complex, robust, peren- 

 nial inhabitant of terrestrial zones. Increased sporogenous 

 tissue was necessarily accompanied with a more bulky struct- 

 ure, which then necessitated a differentiation of its tissue by 

 sterilization of certain external, then internal, parts for protec- 

 tion and circulation. Robust types of land plants could more 

 naturally be developed from such a phase than from the ex- 



