220 The Botanical Gazette. [May, 
the fit? I doubt whether we have as yet any certain knowl- 
edge upon any of these points. Inseparably bound up with 
these questions is that as to the direct effect of the external 
world. Light makes some shoots grow flat: how much 0: 
form and size conditions are thus directly imposed, and hence 
not due to natural selection? How much to internal consti- 
tution or mechanical growth conditions? How much to 
selective action of the environment? The greatest problem 
of the present time in the dynamics of evolution, is the delimi- 
tation of the effects of these three sets of influences: (1) in- 
ternal constitution, which includes (a) properties or functions 
of protoplasm, (4) hereditary characteristics of the particular 
organism, (c) mechanical, physical or chemical conditions of 
growth; (2) the direct effect of external, mechanical, physi- 
cal or chemical influences upon the plastic organism either 
(a) directly, or (4) through the intermediation of irritability; 
(3) the preservation of adaptive variations, and the elimina- 
tion of the unadaptive by the operation of natural selection. 
A few years ago an overwhelming preponderance was ascribed 
to the third, but evidence is accumulating to show that the 
first and second playa part equally, perhaps more, important. 
I doubt if any family of plants can equal the Cactacee, when 
properly studied, in their bearing upon these matters. 
Aside, however, from this, the family is one which the ex- 
treme natural selectionist will find it difficult to deal with. On 
the deserts the conditions of life are comparatively speaking, 
very uniform. Heat and light, the gases of the air, the min- 
erals of the soil are more than ample for all, and all share prac- 
tically alike as to rain. Enemies are not numerous, no 
highly differentiated, and not of the kind with which they 
come into competition for survival. The plants grow well 
apart, and the struggle for existence lacks the complexity of 
forest and jungle and is reduced simply to a struggle wit 
hard but uniform inorganic nature, the scarcity of water being 
the greatly preponderating element. Such conditions as these 
should, upon the natural selection hypothesis, produce a mo- 
notonous, little differentiated vegetation, for without keen 
competition between similar or closely related forms, slight 
favorable differences have no chance of survival over slight 
unfavorable ones, and hence differentiation along advanta- 
geous lines alone could not occur. Now what are the facts? 
Despite uniform, non-competitive conditions, this family has 
