PHYLOGEXY OF AXGIOSPERMS 291 



tion. Attention has been focused so long upon the gametes and spores 

 as the two dominant factors in differentiation that it is hard to con- 

 ceive of the possibility of the domination of another factor. It is 

 entirely conceivable, however, that another form of differentiation 

 may have occurred, dominated by the needs of the chlorophyll work, 

 and not by spore-production. Certainly a great need for change, when 

 aquatic conditions were exchanged for terrestrial, was in connection 

 with the display of chlorophyll tissue. It would seem as if the Bryo- 

 phytes had laid emphasis upon spore-production, and therefore never 

 became organized for the fullest use of terrestrial conditions, while the 

 Pteridophytes laid emphasis upon chlorophyll work and became highly 

 organized for terrestrial life. It would seem possible, therefore, with 

 the three factors to take into account, that two distinct asexual lines 

 may have been organized, distinct in the factor selected to domi- 

 nate. . . . 



If more favorable structures can be developed in response to the 

 needs of spores or gametes, there seems to be no good reason why more 

 favorable structures may not be developed in response to the needs of 

 chlorophyll work. If such a response in structure is possible, it would 

 naturally express itself first in developing the largest display of chlo- 

 rophyll tissue in the most favorable region of the body, which would 

 gradually become differentiated more and more distinctly from the 

 rest of the body. It does not seem clear why the appearance of an 

 erect leafy axis, bearing neither gametes nor spores, is not quite as 

 supposable as the appearance of a sporophore with neither gametes nor 

 leaves, or a gametophore with neither spores nor leaves. . . . 



With such an origin of the leafy sporophyte, it would follow that 

 foliage leaves are not secondary but primary structures, and that sporo- 

 phylls have arisen from the differentiation of foliage leaves bearing 

 sporangia, a state of things certainly suggested by the most primitive 

 Pteridophytes known. It would further follow that the evolution of 

 the strobilus has followed the development of foliage leaves, a view in 

 accordance with the older morphology. Such a view would make 

 intelligible the great " gap " recognized as existing between Bryophytes 

 and Pteridophytes, as the two groups would not be phylogenetically 

 connected, and would have developed along very divergent lines from 

 the first. It would mean that at least two independent sporophyte 

 lines have appeared, the Bryophyte line probably with an antithetic 

 origin, and the Pteridophyte line possibly with an homologous origin. 

 The great prominence of the latter line, with its Spermatophyte 

 sequence, is correlated with the development of a vascular system, and 

 it would seem as though the evolution of an elaborate vascular system 

 must have depended upon the domination of chlorophyll work. 



Knowledge of the various theories as to the origin of species 

 is so much a part of the essential training of the morphologist 



