GREAT STEPS IN ORGANIC EVOLUTION 903 



plant-world has been so rich, and its types are so successfully 

 adaptive. 



Again, in that advancing desiccation of our planet, with increase 

 of dryness even towards desertic conditions over vast areas, we 

 have another potent cosmic factor of evolution, and with a new 

 variety of plant adaptations accordingly. Conversely, too, the 

 development of plants along the range of inland water-margins and 

 of marshes, shows also no lack of appropriate adaptations, as of 

 course also many related to the enrichment and deepening of soils 

 from the decomposition of vegetable growth. Again, experiments 

 have shown varied response and change in embryonic forms, from 

 slight changes in the composition of their surrounding water: and 

 thus we can well imagine that the increasing salinity of the ocean 

 has been a notable factor in the appearance of variations, as well 

 as in the selection of the fittest. 



Between these cumulative changes of environment throughout 

 evolutionary time on the one hand, and on the other the annual 

 changes of the seasons, also so frequently with characteristic life- 

 adaptations, we have also to realise the enormous range of inter- 

 mediate changes, as from the recurrence of glacial periods to 

 the wide range of geologic alterations of the earth's surfaces and 

 levels, and from slow but extensive geologic and geographic changes 

 to sudden catastrophes. So, as the geologic world-phantasmagoria 

 becomes more and more vividly and comprehensively presented to 

 the mind, it is no wonder that palaeontologists and geographers so 

 frequently attach great or even predominant importance to such 

 large environmental changes as significant to the appearance as 

 well as the selection cf variations in the forms of life ; and that they 

 are little moved by arguments against their views even when sup- 

 ported by experiments, since these are necessarily on a scale of 

 magnitude and time infinitesimal in proportion to those of their 

 studies. 



RECURRENT GLACIAL PERIODS AS EVOLUTIONARY 

 FACTORS. — Though Ramsay and others were teaching this 

 recurrence fifty years ago, two recent and notable books of com- 

 plemental viewpoint, geological and climatological respectively, 

 yet in broad agreement, greatly clear up the subject, as has been 

 pointed out in able exposition by Dr. Newbigin. First, by their 

 agreement on at least three major Ice Ages — Huronian, Cambrian, 

 and Permo-Carboniferous — before the recent and best studied 

 Pleistocene one, from whose chilling grasp the world is not yet free. 

 Next, in abandoning Croll's and other astronomical theories, and 

 returning to geologic and geographic ones — of great tectonic 

 uplifts, with differences of relief and of distribution of land and 

 water. 



