30 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



the most mobile of the all, are not infrequently driven by external forces 

 into new regions. The migration of these animals is thus far equally passive 

 with that of plants. 



Turning to plants and attempting to summarize the powers effecting 

 their passive dispersal, we find a large and imposing array. Among others, 

 spores and light seeds are carried by moving currents of air for great dis- 

 tances, heavier seeds are floated, or carried in the intestines of migrating 

 animals, or attached to their bodies, to be dropped at great distances. The 

 chances of survival are neither greater nor less in the new environments 

 than are those of transported animals. To assume that the flora of a region 

 has always reached any particular place by the slow process' of self-seeding 

 under the influence of a slow-shifting climate would be most erroneous. 

 On the other hand, it would be equally dangerous to assume that a climatic 

 change comes slowly or suddenly upon a restricted area and remains to 

 influence it, and that the plants indicate an area of climatic isolation with 

 unchanging boundaries. It has been repeatedly shown how climatic changes 

 advance broadly over wide areas in a slow but irresistible march, and the 

 flora may advance or disappear by self-seeding or death in an equally gradual 

 manner. An excellent illustration of a relatively sudden climatic change 

 has been given by Marais. 1 Typical illustrations of more slow and regular 

 changes are given by Huntington. 2 



The fact that the flora of any period of geological time is frequently in 

 advance of the fauna in its evolution is well known ; the controversy as to 

 the upper limits of the Cretaceous hangs entirely upon the evaluation of the 

 floral and faunal evidence. 3 Other cases of a similar kind are well known. 

 May it not be that slowly advancing climatic changes have permitted 

 plants to advance into new regions, where the greater adaptability of the 

 vertebrate forms permitted older types to persist for a long time in slightly 

 changed conditions? 



The interpretation of the adaptations of plants to environment is in the 

 hands of the botanists, but one example of possible confusion may be cited: 

 Desert plants are protected by a heavy layer of thickened peripheral cells- 

 palisade cells and the stomata are set in deep grooves, protected by hairs 

 or wax, or practically closed. These are adaptations to prevent rapid 

 evaporation, but similar histologic conditions are found in some plants of 

 stagnant swamps. It is suggested that this is to prevent rapid evaporation 

 and so prevent the plant from absorbing too large a quantity of poisonous 



1 Marais, E. N., Notes on Some Effects of Extreme Drought in Waterberg, South Africa, 



Agricultural Journal of South Africa, February 1914. Reprinted in Annual Report 

 Secretary Smithsonian Institution for 1914, p. 511. 



2 Huntington, E., The Pulse of Asia. 



Some Characteristics of the Glacial Period in Non-Glacial Regions. Bull. Geol. Soc. 



Amer., vol. 18, p. 351, 1907. 



1 A Symposium upon the Cretaceous-Tertiary Boundary Line. Papers by Osborn, Knowlton, 

 Stanton, Brown, Matthew, in Bull. Geol. Soc. Amer., vol. 25, pp. 321-402, 1914. 



