472 MR. H. B. GUPPY ON PLANT-DISTRIBUTION. 
. 
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 Podostemace:e. 
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 differentia- 
tion of the climatic zones. Behind their disregard for the present arrange- 
ment 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 isa 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 Dico- 
tyledons display a much greater tendency to detachment from the tropics 
than the Monocotyledons, the Sympetale 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. 
