Islands in Favour of the Age and Area Theory of Willis . 517 
Labiate genus. Of the 20 Canarian species 15 are confined to one island, 
3 to two islands, 1 to three islands, and the last, M. varia, a highly variable 
species, abounds all over the group, occurring also in Madeira. They 
assume that M. varia has probable descendants in species restricted to 
single islands and they extend this position to many other genera. We 
could not admit (they argue) in the cases of numerous other genera, Statice, 
Senecio^ Sonchus , &c., where this occurs, that the species originally existed 
in all the islands and died out in all but one. Evidently they hold with 
Hooker that the single-island species have arisen as adaptations to the 
particular conditions of individual islands. 
We have here spontaneous testimony that the role of the polymorphous 
or highly variable species in the development of new forms, a role which 
is so conspicuous in the later floral history of the islands of the tropical 
Pacific, is equally well illustrated in the Canaries. The principle is 
exemplified in its simplest shape in a compact genus or subgenus ranging 
over an archipelago and holding a score or so of species. Here a solitary 
highly variable species ranging over all the islands becomes the parent of 
several localized species that are often confined to single, islands. But it is 
as true of a section of a genus characteristic of one group of islands as it is 
of a large genus distributed over many archipelagos and holding, as in the 
case of some genera of the Pacific, an ocean in its sway. It is as true of 
a subgenus confined to a small continental area as it is of a wide-ranging 
genus that covers a continent. It is often beautifully illustrated in the beha- 
viour of a single species as a parent of numerous local races and varieties. It 
was the part taken by the polymorphous species in the Pacific, differentiating 
in every group and even in the individual islands of a group, that first led 
the writer to view the world of Nature as in the main a differentiating world. 
It represented for him in miniature a fundamental principle of distribution. 
Here on a small scale he recognized the process of the differentiation of the 
primitive generalized types that once ranged the globe. 
The subject was first worked out by the writer in his volume on plant 
dispersal in the Pacific ( 1906 ), and it is there shown how numerous 
botanists, however much they might differ in other points, were at one 
in recognizing the play of the polymorphous or highly variable species ifi 
that region. The principle is either illustrated or implied in the case of one 
genus or another in the writings of most of the botanists who were interested 
in the floras of the Pacific Islands during the latter half of the last century 
and in the beginning of this, of Bentham, Burkill, Cheeseman, Drake del 
Castillo, Gray, Hemsley, Hillebrand, Reinecke, Rendle, Seemann, and 
others. Now we find it recognized in the Atlantic Islands, directly in the 
pages of Pitard and Proust, and by implication by Hooker for the Canaries 
in his reference to the ‘ generally accepted views ’ respecting the origin of 
localized endemic species in that group. 
