xciv PROCEEDINGS OF THE GEOLOGICAL SOCIETY, [vol. lxxix, 



gradually lost the impress of a common parentage. A well-known 

 example of such a generalized type used to he the so-called Seed- 

 bearing Ferns or Pteridosperms, which were a prominent 

 feature in the forests of the Coal Period. It was believed that 

 the Fern phylum was the source of all Seed-plants. The majority 

 of us, in different degrees, shared this belief. Opinion has now 

 altered. Dr. D. H. Scott, who formerly supported the old view, 

 has changed his mind — men who never change their minds are, as 

 William Blake quaintly but truthfully expressed it, like stagnant 

 water, and ' breed reptiles of the mind '. Dr. Scott has recently 

 written : — 



' The inference from all the facts at present available appears to be that the 

 Seed-plants, of which the Pteridosperms are among the earlier representatives, 

 constitute an independent phylum, of equal antiquity with any of the recog- 

 nized lines of Vascular Cryptogams.' 



The Middle Devonian Asteroxylon, its stem-anatomy and habit 

 essentially Lycopodiaceous, its sporangia more akin to those of 

 Ferns, might also be quoted as a generalized type suggesting a 

 common origin for the two groups ; on the other hand, its Fern- 

 like sporangia may merely illustrate a variation from the normal 

 Lycopodiaceous pattern, reproducing a type of construction adopted 

 as a permanent possession by the independently evolved Fern 

 alliance. Dr. Church, in his Essay on ' Thalassiophyta & the 

 Subaerial Transmigration,' lays stress on the isolation of the 

 modern representatives of the great sections of the plant-kingdom, 

 and contends that schemes of linkage between the different lines 

 of development are wholly fallacious. All the various lines of de- 

 velopment of what is now land-flora, he believes, must have been 

 'differentiated in the Benthic epoch of the sea {i.e., on algal lines)', 

 as all algal lines were differentiated in the Plankton phase. The 

 occasional occurrence in one type of plant of features characteristic 

 of different groups is, in itself, no adequate reason for assuming 

 community of descent. In the earlier, more experimental stage of 

 evolution, the plasticity of plants belonging to different lines of 

 development would inevitably sometimes find similar expression. 

 The potentiality of protoplasm is the determining factor, and it 

 Avould seem more logical to picture the unfolding of the several 

 groups of the plant-kingdom as so many distinct processes governed 

 by the forces enclosed within the cells of diverse marine proto- 

 types, than to assume the wholesale destruction of common 

 ancestors demanded by those who prefer to link into one complex 

 of infinite resource the several sections of the plant- world. 



