SUMMARY
From a consideration of the problems of plant-distribution, the writer is led to regard the history of the Angiosperms as resolving itself into two principal eras:— The era that witnessed the rise of the great families, a period of relatively uniform conditions.The era that witnessed the differentiation of these family types in response to the differentiation of the climatic and other conditions.
It is argued that conclusions drawn from the prevailing influences now in operation could only be applied to the differentiation of the ancient family types–that is to say, to the second era in plant-history. It is not possible, so it is held, to apply a theory based on the present to an age of other things, other ways, and other conditions. Only the hypothesis that finds its guide to the past in the abnormalities of the present can be of service to us in the interpretation of times so different.
The subject is introduced by a reference to two papers, contributed to the ‘Journal of the Linnean Society,’ which have an important bearing on the subject, the one by Bentham on the Compositæ, the other by Huxley on the Gentians. Then follows a statement of the differentiation hypothesis which involves the differentiation of primitive world-ranging types in response to the progressive differentiation of their originally uniform conditions. Allusion is then made to the dilemma into which all theorists fall when they come to handle the larger groups, the very persistence of which in our own age depends on the stability of their essential characters. If stable now, why so unstable then? We are thus forced to the conclusion that in the distant era that witnessed the deployment of the Angiosperms instability prevailed. It was an age of mutations, free and unchecked, and an age of uniformity of conditions, the mutability decreasing and the modifications becoming more and more fixed with progressive differentiation of conditions, an explanation suggested by a perusal of the accounts by Dr. Willis of his prolonged investigation on the Podostemaceæ.
The distribution of families is then treated statistically; and it is shown that whilst they largely ignore the cleavage of the land into two great masses diverging from the north, they respond in a marked degree to the differentiation of the climatic zones. Behind their disregard for the present arrangement of continents and oceans lies the story of the first era, and behind their ready response to climatic differentiation lies the story of the second era. In the circumstance that the response made to the bi-cleavage of the land-mass is absent or small with the larger groups and becomes greater and greater as we go down the differentiating scale until it attains its maximum in the species, is recognised the contrast of conditions between the pre-differentiation era and the era when differentiation reigned supreme. It is held that there is a method here disclosed that could only arise by the family differentiating into the tribes, the tribe into the genera, and the genus into the species, since the opposite method of commencing with the species would produce chaos.
The paper ends with the application of the statistical treatment to the larger groups behind the families, and it is shown that whilst the Dicotyledons display a much greater tendency to detachment from the tropics than the Monocotyledons, the Sympetalæ stand foremost in this respect amongst all the groups of the Dicotyledons. It may be added that there is a large amount of material in the ten tables which from considerations of space could not be discussed. These data have therefore to tell their own story
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