248 Aspects of Ecology. 
development of the quasi-organism. Dr. Clements’ discussion of 
this part of the subject is characterised by close and apparently 
exhaustive logical analysis, most of which is original and much of 
great value as laying down fundamental principles in a subject that 
has suffered in the past from lack of this kind of treatment. 
It is impossible in the space at our disposal to criticise this 
closely knit exposition in detail. While, as we have said, much of it 
is of great excellence, it is not wholly free from doubtful statements, 
nor, in parts, from a tendency to over-elaboration. As an instance 
of the former we may take the following sentence (p. 208), 
“Accepting the easily demonstrable fact that an excess of salts in 
the soil water, as well as cold, tends greatly to diminish the available 
water of the soil, i.e., the chresard, it is at once seen why saline 
and arctic plants are as truly xerophytic as those that grow on rocks 
or in desert sands.” This statement assumes the validity of a 
simple explanation of a phenomenon which is by no means 
thoroughly understood. The hypotheses that have been put forward 
by Schimper and others have, so far as we know, not been experi¬ 
mentally tested, and it seems almost certain that the conditions 
involved are not susceptible of explanation by a simple conception 
such as that of Dr. Clements. 
The concepts of migration, ccesis (the becoming established of 
migrants), reaction (of the habitat on the invaders) and competition, 
as successive processes in the phenomena of invasion and succession, 
which are fundamental in the development of vegetation, are 
valuable as furnishing an analysis of this important and rather 
neglected field, but the treatment in some cases appears to amount 
to little more than a somewhat lengthy enumeration of special 
cases. 
The treatment of Barriers, of Endemism, and of Polyphylesis 
and Polygenesis is good and helpful. The distinction drawn 
between the two last is clear and logical, while the strong 
probability of the occurence of both in nature is clearly shown. 
It is plain that if in polyphylesis the convergence is sufficiently 
great it is impossible to deny “ naturalness ” to the resulting 
aggregate, whether species or genus. This whole subject is 
however complicated by our ignorance of the origin and nature of 
hereditarily constant species, whether adaptive or not, for of some of 
the examples cited by Dr. Clements it might be asserted that, 
though commonly known as species, they are not really so, but 
only ecads with marked structural differences. Convergence in 
such a case would not be real polyphylesis. 
