ORIGINATION OF SELF-GENERATING MATTER 621 
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 réle. 
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. 
