220 
Two Recent Ecological Papers. 
gives, of course, a semi-desert climate. Most of the rain, however, 
falls from April to September, and this gives rather a luxuriant 
summer growth. The relative humidity ranges very low—even as 
low as 1% during the day in some cases,—the averages being round 
50%, and saturation, giving a deposit of dew, is quite rare. There are 
no very extreme temperatures, but the daily range in winter is very 
considerable, while the difference between the daily temperature- 
curves of soil, soil surface, and different heights above the soil, are 
very striking. Exposure is of very considerable importance in 
altering the physical conditions. The physical factor records are 
taken with the elaborate thoroughness characteristic of Professor 
Clements’ school. 
The formation covering the Mesa is the Bouteloa (grama-grass) 
formation in which Bouteloa oligostachya is the dominant species. 
Clements’ terms consocies (corresponding to what is called an 
association in this country), society and community arc used for 
various divisions of the formation, hut it would have been useful if 
the author had devoted a little space to a discussion of the relations 
of these units, illustrated by the special phenomena with which he 
is dealing. Some of the conceptions involved are still unfamiliar to 
European ecologists, and that most desirable end, uniform nomen¬ 
clature, which Professor Clements has so much at heart, can only 
be attained by repeated and convincing evidence of the soundness 
of the units adopted. 
The formation is described under prevernal, vernal, festival and 
autumnal aspects, an important and indeed essential recognition of 
seasonal change, which is often neglected, but nothing is said about 
the “ consocies ” of the formation in spring and autumn, though the 
“ consocies ” are said “ at all periods to give the characteristic 
stamp to the vegetation,” while the societies and communities are 
only found in each “aspect.” This is probably no more than a 
formal omission, but it makes this part of the paper rather confusing 
to read. A number of “ list quadrats ” of the various consocies 
and societies are given with the soils and water-contents for the 
different seasons or “aspects.” These are very useful as indicating 
the kind of limits of water-content—usually the determining 
factor—under which the different species can grow and become 
abundant. An analysis of the figures to bring out these limits more 
exactly would have been welcome, as also some “chart-quadrats” 
to illustrate further the structure of the formation. The critical 
determining factors of the different units of vegetation is, of course, 
the most important object of exact physical factor determinations, 
