xlvi 



INTRODUCTORY REMARKS. 



may be in reality but a depauperated phasis of the former, conse- 

 quent upon an alteration (at some distant period) in the various 

 local influences by which the species was originally surrounded. 

 And I might again appeal to the Land-shells, in support of this 

 hypothesis ; for there we have ocular demonstration, in the deposits 

 above referred to, that certain gigantic forms, now well nigh extinct 

 but which absolutely teemed in those early days, are represented at 

 the present time by others which are equally common, only reduced 

 to about half the size. Yet there is no case here of a gradual trans- 

 mutation ; for the intermediate links do not exist, even though both 

 forms are found under both conditions the smaller ones being as 

 rare semifossilized as the larger ones are recent The transition 

 from one state to the other appears to have been sudden, as though 

 occasioned by some radical change in the physical conditions of the 

 area overspread ; and the result is now before us, in the deteriorated 

 race occupying the deteriorated region. 



There are several other points, in connexion with this immediate 

 subject, to which I might properly call attention ; but space will not 

 permit me to do so. A few words, however, I will just add on the 

 general character of the fauna. That the eastern parts of the 

 Canarian, and even (though less decidedly) of the Madeiran, Group 

 appear to have much in common with the sandy districts on the op- 

 posite coast of Morocco, I have already expressed my belief ; yet, 

 in spite of this, I think that a truly " African " element is perhaps 

 scarcely indicated. Nearly all the species which are not absolutely 

 peculiar to the islands seem to be (what would be termed) " Me- 

 diterranean " being found (more or less) on one side or both of the 

 Great Mediterranean basin, and extending down the western limits 

 of Barbary and Morocco, but not (so far as I can ascertain), on the 

 central African continent, south of the Atlas range. Yet, at the 

 same time, there is reason to suspect that so large a proportion of 

 the forms are positively endemic, that to define the fauna as simply, 

 and purely, Mediterranean, would be wanting in accuracy ; for the 

 most significant, and esoteric, genera do so thoroughly permeate the 

 entire archipelago (represented, however, on the different islands by 

 different species) that the unity of character which they impart to it 

 is perhaps more suggestive of a separate " Atlantic province," than 

 of a component part of the quondam " Mediterranean " area. 



With respect to the Groups themselves, so completely do they 

 seem to constitute (when combined) a single system, that, be the 

 geological difficulties what they may, I must be excused if I have 







