Aspects of Ecology. 249 
The general treatment of “ Invasion ” and the long section 
devoted to “ Succession ” are most excellent and interesting; the 
latter, with its descriptions of numerous examples, evidently gains 
greatly from the author’s studies on the Colorado mountain 
vegetation, to which we have already alluded. The talus or 
“ gravel slide ” succession in which a final Picea-Pscudotsuga forest 
arises as the last term of a series of seven stages of development, 
is a particularly extensive and complete example of fully studied 
vegetation development. 
Under “The Structure of the Formation ” Dr. Clements first 
considers the principles of zonation and alternation, to which all 
the structures actually found are due. The concept of zonation is 
the oldest in phytogeography, vegetation-zones having been 
recognised by Tournefort in 1717, but apparently no general 
analysis has hitherto been attempted. “ Zonation ”, says our 
author, “ is the practically universal response of plants to the 
quantitative distribution of physical factors in nature.” It would 
perhaps be more accurate to say that it is the response to regular 
spatial quantitative change of the physical factors, wherever such 
change occurs ; and the extremely widespread and constantly 
recurrent existence of this type of distribution is the cause of the 
almost universal occurrence of zonation. The detailed treatment 
of this subject is again excellent. 
The other general principle governing formational structure 
and distribution is that of “ alternation”, which was first stated by 
Dr. Clements in his “ Development and Structure of Vegetation ” 
(1904). It is “ that phenomenon of vegetation in which a formation 
recurs at different places in a region, or a species at separate 
points in a formation.” “ Alternation is the response of vegetation 
to heterogeneity of the earth’s surface.” It is in such cases the 
expression of the breaking up of the earth’s surface into different 
types of habitat and obtains everywhere within the major zones of 
the globe in r.reas of different rank down to the “patches” found 
in the smaller units of vegetation while heterogeneity of habitat is 
one great cause of alternation, there is another cause which 
Dr. Clements seems to neglect in his general discussion though he 
recognises it in his treatment of the sub-divisions of the formation 
(p. 295). That is the historical cause, the occurrence of a species 
or group of species in one part of a given uniform habitat, and not 
in another, through historical accident, simply because certain 
plants arrived at a certain spot first and other plants at another, 
