The British Association at Winnipeg. 367 
On Friday morning Dr. Armstrong communicated a paper by 
Professor Willstatter, on the “ Chemistry of Chlorophyll.” 
Professor Willstatter’s work is of course well known, but it was 
interesting to have a resume presented to Section K. The paper 
laid stress on the differences, rather than the resemblances, between 
the respective pigments of blood and green leaves. While the 
molecule of haemoglobin contains iron as its essential metal, in that 
of chlorophyll the iron is replaced by magnesium. 
In contrast to the Dublin meeting, where physiology was so 
strongly represented, this was the only definitely physiological paper 
communicated. On the other hand, several of the ecological 
papers, which formed the chief feature of Friday morning’s sitting, 
dealt with the physiological aspect of ecology. 
Professor H. C. Cowles (of Chicago) led the way with a most 
interesting paper on “ The Fundamental Causes of Succession 
among Plant-Associations.” The author pointed out that ecological 
plant-geography is emerging, just as taxonomy and morphology 
have emerged, from the merely descriptive stage, and that the 
genetic relationships among plant-associations are now claiming 
attention. Recent work on the life-histories of these associations 
show that in general “ edaphic formations ” tend to develop into the 
“climatic formations” of the particular region. For this reason 
the author suggests the disuse of the terms edaphic and climatic, 
and their replacement by such terms as proximate and ultimate, 
which explicitly imply evolution. Intermediate stages between the 
proximate and ultimate or final, may be designated approximate, 
mediate and penultimate. In exceptional cases, where the external 
conditions may remain uniform for long periods of time, e.g., in 
deserts, there is practically no succession, and the proximate and 
ultimate formations are the same. But in most cases, there is a 
sufficient, if gradual, change of conditions, and therefore succession 
is the rule. Even where the topography and the climate remain 
stable, the plants themselves gradually bring about such changes of 
conditions. Thus the accumulation of humus alters the water- 
content of the soil; increasing it, for instance, in a sandy upland soil, 
and decreasing it in low-lying, swampy soil. The increase of humus 
also materially affects the aeration, temperature and food-supply of 
the soil, and favours the growth of saprophytic fungi. A second 
important factor in succession is the influence of increasing or 
decreasing shade. Proximate upland associations are exposed to 
maximum illumination, while each succeeding stage is characterised 
