1988 The Zoologist— July, 18C8. 



but, in instances where to kx\ov is the special end to be attained, even 

 imagination (in its highest sense). 1 need scared)' add that a denial 

 of this Supreme power as inherent in 'nature' is perfectly com- 

 patible with a belief in those modifying external influences which 

 all experience assures us are ever liable to act, within reasonable 

 limits, and to leave their impress, upon organic structures, in 

 accordance with the exact amount of pliability which has been 

 allotted to each separate species ; for this is totally distinct from 

 that selective capability which we are accustomed to regard as an 

 integral part of free agency and will. Mere variation we all know 

 to be a fact; and, if its importance is by some exaggerated, no one 

 has ever yet questioned its existence ; but I believe it can seldom be 

 said to 'accumulate' during more than a few generations, or ever to 

 go on increasing in an undeviating course after the effect has been 

 accomplished which is legitimately due to the combination of circum- 

 stances which occasioned it. Towards the close of my introductory 

 observations in the ' Coleoptera Atlantidum,' I cited the Madeiian 

 land-shells in support of this thesis, showing that, so far at least as 

 they are concerned, we possess ocular demonstration that they have 

 not altered during the enormous interval which must have elapsed 

 since the commencement of their subfossil era, except that the size of 

 a few of them appears to have been suddenly reduced (for there are no 

 traces of the intermediate grades of stature, which must have been 

 preserved under any process of a gradual dwindling down), as though 

 consequent upon some physical catastrophe or depauperation, in the 

 areas over which they had spread; and this, supported by other con- 

 siderations, led me to infer that the many trifling insular departures 

 which we meet with, from a central type, were not, in all probability, 

 brought about by any slow and imperceptible method of long-cou- 

 tinued, cumulative change, but in a comparatively short period 

 (terminating when the natural conditions of the newly. acquired 

 habitats had ceased to alter), and perhaps through the partial breaking 

 up of this vast Atlantic province. At least some such inference seems 

 borne out in many ways, and to accord with the two-fold fact that, 

 while these trifling insular aberrations are everywhere conspicuous, we 

 have at the same lime most unmistakable evidence of what I may 

 almost call the unchangeability of a large proportion of the present 

 forms. And although it is true that my remarks arose out of 

 Madeiran data, I am satisfied that they are equally applicable to the 

 whole of these sub-African oceanic groups." — Introduction, p. xxxvi. 





