GREAT STEPS IN ORGANIC EVOLUTION 871 



how the transition from aquatic to terrestrial (or subaerial) condi- 

 tions may have been effected. 



Another contribution to the problem of the origin of land plants 

 has been given by the distinguished Oxford botanist, Dr. A. H. 

 Church, in an essay entitled Thalassiophyta and the Subaerial 

 Transmigration (Oxford University Press, 1919), an essay as full 

 of suggestive ideas as it is of difficult terms. Dr. Church's general 

 idea is that terrestrial plants arose by the gradual transformation 

 of highly evolved marine plants on a slowly rising beach. Transmi- 

 gration for him means "transition in situ". "When the first land 

 gradually lifted above the primal sea, bearing all forms of marine 

 life on it, the successful transmigrant algae of the first land migration 

 combined the best and highest factors of marine equipment." What 

 had been gained in the sea in the course of ages was not lost, to be 

 invented de novo a second time; it was adapted. It was not in the 

 reproductive part of the plant that the profoundest changes were 

 necessary; it was the body that required to be readjusted, from life 

 in an aqueous food-solution to life in an atmospheric medium, with 

 no external food-solution beyond that absorbed by the roots. 



After the gradual cooling of the earth there were, according to 

 Dr. Church's picture, three great epochs of world-construction, with 

 associated vegetations. There was the time of the condensation of 

 water-vapour to form the sea, which he supposes to have covered 

 the earth, and the surface-waters of that sea were peopled (as still 

 so much to this day) by microscopic plants sufficient unto them- 

 selves. This was the Plankton Epoch. Second, the folding of the 

 earth's crust raised parts of the floor of the sea within the reach of 

 light, and minute plants began to settle there, anchoring themselves 

 and proceeding to build up fronds and other forms of body. But 

 anchoring on a substratum made it necessary to have some new 

 arrangements to secure dispersal — a return to the plankton phase 

 for processes of reproduction, much in the same way as we see in 

 sponges which liberate free-swimming embryos, or in zoophytes 

 which liberate swimming-beUs or medusoids. A new note was struck : 

 the types that survived were those whose individual members had 

 moved in the direction of race-continuance — that most fundamental 

 of all biological truisms. To the plankton law of self-preservation 

 was added "the benthic law" of race-continuance. "The fact that 

 any race still exists implies that the individuals collectively have 

 done their bit." This was the Benthos Epoch, i.e. of plants still 

 submerged. Third, there was the gradual emergence of land plants 

 and the gradual transformation of aquatic vegetation — seaweeds 

 for short — into a land flora, able to absorb gases from the air and 

 salts in solution from the moist substratum. The Benthos life had 

 introduced the new factor of substratum, but the emergence of the 



