296 D. T. MACDOUGAL 



place at the present time, and their accomplishment experimentally 

 is attended with difficulties not yet surmounted. 



The part of the evolution of living matter which may be brought 

 under observation in living forms or preserved material represents 

 very advanced stages and the cell is separated by a wide range of 

 development from the colloidal masses in which self-generating 

 matter first took form. The construction of substances which might 

 use or transform energy other than that of chemical structure represents 

 the first differentiation between the animal and vegetable organisms. 



Plants were necessarily confined to aquatic or hygrophytic habitats 

 as long as their history included the free gametophyte, and a land 

 flora became possible only with the development of the sporophyte 

 culminating in the derivation of the seed-plants. In this and in 

 subsequent history the water-relation has played the predominating 

 morphogenic role. 



The desiccation of a region occupied by a land flora would entail 

 a complex series of changes in climatic and other environmental 

 factors which may be followed by extermination or differentiation 

 of the flora. This differentiation, which would ensue most readily 

 in regions of diversified topography, with an absence of barriers 

 preventing distributional adjustments, would include localizations of 

 habitats, seasonal restriction of activity, and transformation of the 

 competitive struggle for existence from one chiefly among plants to 

 one between plants and animals. 



The surviving flora would include an important proportion of 

 mesophytes or hygrophytes, while the arid conditions might be 

 followed by the development of two types of xerophytes, succulents, 

 and spinose forms. 



The amelioration of desert conditions would mean a reversal or 

 shifting of various environic factors, the whole favoring the increase 

 in the number of individuals representing the mesophytes, the 

 widening of their habitats, and the institution of the fiercer compe- 

 tition among plants. Such changes would force a retrogressive 

 development on the xerophytes, exterminating many, restricting the 

 range of all, and would result in the survival of a few under con- 

 ditions wholly foreign and antagonistic to those in which their 

 characteristic qualities originated. 



