Gradual ’ Change , and by Guppy s Method of Differentiation. 613 
the primary group of these plants as the least differentiated, he alludes to 
the strange general parallelism in the northern hemisphere between the 
gentians and the crayfishes. It is the case of the tapirs over again, he 
also adds.’ 
‘ If we take the history of distribution of such families as the Menisper- 
maceae and the Monimiaceae [cf. ‘Age and Area ; , p. 172] we have the story 
of the dissociation of two family types into tribes, genera, species, and 
varieties, and as the parent type differentiates into tribes, and the tribes into 
genera, and the genera into species, varieties, and races, a subsidiary prin¬ 
ciple which has been termed “ Rank and Range ” comes into view. Here 
range goes with systematic rank, and since age goes with it as well, we have 
the Age and Area principle exemplified. 
‘ But the whole story of the distribution of the flowering plants since 
their appearance in secondary times has been a story of differentiation of 
types and the dissociation of floras. From the migration standpoint this 
has long been studied by Mrs. Reid. But what I am concerned with here 
is the dissociation of the mixed floras of the Eocene ages, the elements of 
which are now separated in different climatic zones, and the breaking up of 
the synthetic pro-angiosperm types of the Cretaceous ages, types that com¬ 
bined characters that are now far apart both in a systematic and in 
a geographical sense. The phrase of “ the age of palms and poplars r ’ has 
been applied to the mixed Eocene floras, but the same forces of differen¬ 
tiation that in recent ages have placed palms and poplars in different 
climatic zones previously broke up the comprehensive pro-angiosperm types 
of the Cretaceous period, types that ushered in the appearance of the Dicoty¬ 
ledons, which now present the characters of the primary parent types in 
a dozen different families systematically and geographically separated from 
one another. The dissociation of mixed floras, and the differentiation of 
synthetic or comprehensive types, have worked together to bring about the 
present system of plant distribution. 
‘ It is noteworthy that Weismann, after supplying the machinery for the 
building up of types, provided the machinery for breaking them up. Con¬ 
comitant variation of many individuals in the same direction and under the 
same stimulus of changing environment furnished the type which under 
the influence of varying conditions would break down into lesser groups. 
‘ After all, differentiation comes nearer home to us than the Darwinian 
machinery of evolution. It has influenced in the past, and is still in the 
present influencing, our amusements, our arts and sciences, and even our 
creeds. It is a more comfortable doctrine than that involved in the belief in 
the ruthless struggle of existence that is based on the triumph of the strong 
and the crushing of the weak.’ 
To return to my own work, in a later paper ( 15 ) I worked at the 
problem of the distribution of the Dilleniaceae, upon the assumption that 
mutation was the true explanation of evolution. I showed that there was 
a general tendency in the larger genera, here as in other families, for there 
to be one or two widely ranging species, accompanied at various spots by 
local endemics; and I pointed out that the easiest explanation was to 
suppose the former to shed the latter. On this supposition, it was clear that 
