In the following remarks on the history of the indigenous fauna and flora of the British Islands and the neighbouring sea, I take for granted the existence of specific centres, i.e., of certain geographical points from which the individuals of each species have been diffused. This, indeed, must be taken for granted, if the idea of a species (as most naturalists hold) involves the idea of the relationship of all the individuals composing it, and their consequent descent from a single progenitor, or from two, according as the sexes might be united or distinct.
That this view is true, the following facts go far to prove. 1st. Species of opposite hemispheres placed under similar conditions are representative and not identical 2nd. Species occupying similar conditions in geological formations far apart, and which conditions are not met with in the intermediate formations, are representative and not identical. 3rd. Wherever a given assemblage of conditions, to which, and to which only, certain species are adapted, are continuous—whether geographically or geologically—identical species range throughout.
I offer no comments on these three great facts, which I present for the consideration of the few naturalists who doubt the doctrine of specific centres. The general and traditional belief of mankind has connected the idea of descent with that of distinct kinds, or species, of creatures; and the abandonment of this doctrine would place in a very dubious position all evidence the palæontologist could offer to the geologist towards the comparison and identification of strata, and the determination of the epoch of their formation.
Moreover, it is notorious that the doctrine of more than one point of origin for a single species, and consequently more than one primogenitor for the individuals of it, sprung out of apparent anomalies and difficulties in distribution, such as those which I am about to show can be reasonably accounted for, without having recourse to such a supposition.
Having assumed the doctrine of specific centres as true, the problem to be solved is, the origin of the assemblages of the animals and plants
