Islands in Favour of the Age and Area Theory of Willis. 521 
The great climatic revolution that overwhelmed Northern Africa destroyed 
the old Tertiary continental forests of this archipelago. The only con- 
nexions with the northern groups that we should look for here would 
belong to what Christ terms ‘ xerothermic * types that are usually North 
African and Mediterranean in their range. This is pretty much what we 
find ; but we cannot exclude these islands from the floral region. They 
must have belonged to it in the past, and they belong to it now, but in 
a limited sense. They come into it on a lower grade, and must be 
correlated with the lower or African zone of the Canaries and Madeira, 
both in a climatic and in a floristic sense. But the connexion with the 
northern groups, as Christ points out, is not merely a matter of identity 
of species. It is also concerned with affinity. There are a score or so of 
species nearly related to Canarian and Madeiran plants and of similar 
‘xerothermic’ types (Hooker in his Lecture, p. 16, takes a similar view). 
In framing the above list the works before named of Pitard and Proust 
and of Christ have been mainly used. But other sources of information 
have been consulted, such as Schmidt’s ‘Cap Verdische Flora’, 1852; 
Coutinho’s ‘ Catalogus Herbarii Gorgonei’, 1914-15; Watson in Godman’s 
; Azores’, 1870; Trelease on the Azores in the 8th Report of the Missouri 
Botanical Garden, 1897 ; and Lowe’s ‘ Manual of the Flora of Madeira’, 
1857- 
Principal Works consulted. 
Christ, H. : Vegetation und Flora der Canarischen Inseln. Engler’s Botanische Jahrbiicher, 
*Bd. vi, 1885. 
— : Spicilegium Canariense. Ibid., Bd. ix, 1887-8. 
Drude, O. : Handbuch der Pflanzengeographie, 1890. 
Engler, A. : Versuch einer Entwickelungsgeschichte der Pflanzenwelt, 1879-82 (Macaronesian 
floras), i. 74. 
Guppy, H. B. : Observations of a Naturalist in the Pacific, vol. ii, 1906. (See Index under Poly- 
morphous Species.) 
: Plants, Seeds, and Currents in the West Indies and Azores, 1917 (chaps, xvii- 
xix, on the Azorean, Canarian, and Madeiran floras, and chap, xiv, on Polymorphous 
Species). 
Hooker, J. D. : Lecture on Insular Floras. Brit. Assoc., 1866. 
: Marodco and the Great Atlas, Appendix E on Canaries, 1878 (a joint work with 
J. Ball). 
Pitard, J., and Proust, L. : Les lies Canaries, Flore de l’Archipel, 1908, embodying the later 
results of Bolle and Bornmiiller. 
