4 Bews. — Some General Principles of Plant Distribution as 
place to cooler conditions, the retreat to the south began ; and the plants, 
as the diverging continents pulled them more and more asunder, became 
more and more distinct from each other. . . . When the warmer conditions 
returned, the plants advancing northwards met again in the common 
gathering ground around the Pole, but modified by their different experi- 
ences in southern regions lying oceans apart. There they mingled together, 
the eastern and the western floras, and when, with the next climatic change, 
they began again to retreat to their ancient home in the warmer latitudes of 
the south, the east had borrowed from the west, and the west from the east. 
The secular changes of climate have therefore tended in this way to mix 
together the floras of the globe.’ 
Recently, Willis, in a series of papers (37-44), has developed what he 
calls the ‘ age and area ’ law, which is based on the fact that endemic 
species in any country tend, on the whole, to have a narrower range than 
non-endemic species. The older a species in any country, the wider is its 
range. In South Africa, where the climatic conditions are very diverse and 
very mixed even in comparatively small areas, the action of this ‘ age and 
area’ rule is greatly modified, yet, in a general way, it seems to hold and it 
fits in with Guppy’s theory of differentiation. ‘ Differentiation and decrease 
of range go together.’ The theories of Guppy and Willis have been applied 
very successfully by Small (32), in his detailed monograph on the Compositae. 
Most botanists since Darwin have followed him in dismissing the 
possibility of multiple origins. The most prominent exception is Engler, 
who argues in favour of polyphylesis, as he calls it (21). Drude (18) also 
accepted the possibility of multiple origins for major groups. Clements (15) 
has discussed the matter fully, and he should be consulted for further 
references to the literature on the subject. He says, 4 The idea of poly- 
phylesis as advanced by Engler contains two distinct concepts : (i) that 
a species may arise in two different places, or at two different times, from 
the same species, and (f) that a genus or higher group may arise at different 
places or times by the convergence of two or more lines of origin ’. 
Clements accepts Engler’s views and proposes that the term ‘ polyphylesis 5 
should be restricted, as its meaning would indicate, to the second concept, 
and that the term ‘ polygenesis ’ (first suggested by Huxley in the sense of 
polyphylesis) should be used for the first. 
Clements regards the occurrence of different ‘ habitat forms ’ of the 
same species, e.g. of Galium boreale or Aster levis , in different areas, but 
obviously related to the area of the parent form, as proof of polygenesis, 
and he finds further convincing evidence in de Vries’s mutations. ‘ The 
evidence is conclusive that the same form may arise in nature or in 
cultivation, in Holland or in America, not merely once, but several or many 
times.’ ‘ In the presence of such confirmation, it is unnecessary to accu- 
mulate proofs.’ Clements also considers that de Vries’s work proves the 
