402 T. Bland on the Geographical Distribution of Mollusca. 



pothesis, '' that the animals of the respective continents, however 

 near their affinities may be, are in every case specifically distmct 

 from each other," he observes : — 



" The qnestion of the identity of these closely allied species 

 must eventually be decided by their anatomy, but in the mean 

 time we believe it to be perfectly safe to adopt this axiom, that 

 species, whencesoever derived, possessing the same characters, are 

 identical. We view this to be a more rational course than to 

 consider them to be the analogues of each other, a convenient 

 but indefinite mode of expression, which may be used to cover 

 every degree of similitude from a general analogy to a close af- 

 finity hardly admitting of distinction." — p. 76. 



We are quite alive to the danger which exists of the abuse, by 

 naturalists, of the hypothesis which Dr. Biimey deprecates, but 

 we must not argne against its use, from its abuse. 



Jndging from the general tenor of Dr. Binney's work, we do 

 not hesitate to assert onr belief, that had he lived to pursue the 

 subject further, he would not only have concurred in the views 

 expressed by Dr. Gould, and entertained by him in common with 

 many of the most eminent men of the day, but himself have 

 largely contributed to our knowledge of analogous species. 



Our limits do not permit us to enter as fully into the question 

 of the origin of species, as its very important connection with 

 that of geographical distribution demands. 



Professor Forbes, in his before-mentioned paper in the '' Me- 

 moirs of the Geological Survey of Great B?itai?i,^^ takes for 

 granted, at the outset, " the existence of specific centres, i. e., of 

 certain geographical points from which the individuals of each 

 species have been diffused." He adds: — 



" 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." — p. 336. 



Again, he writes, " My main position may be stated in the ab- 

 stract as follows, viz., the specific identity to any extent, of the 

 flora and fauna of one area with those of another, depends on 

 both areas forming or having formed, part of the same specific 

 centre, or on their having derived their animal and vegetable popu- 

 lation by transmission, through migration, over continuous or 

 closely contiguous land, aided, in the case of alpine floras, by 

 transportation on floating masses of ice." — p. 350. 



Lyell, in his Principles of Geology, refers most of the excep- 

 tions to the general rule, that distinct groups of species occupy 

 separate regions, to " disseminating causes now in operation," and 

 proposes the following hypothesis: 



" Each species may have had its origin in a single pair, or indi- 

 vidual, where an individual was sufficient, and species may have 



