1864.] 



Proceedings pf the Asiatic Society. 



341 



not contemplated by Mr. Blanford in his former objections to Mr. 

 Theobald's deduction. The argument was that in two distinct drainage 

 basins, the majority of the species were distinct, whereas one, the 

 pickerel, was common to both, and the inference drawn was that 

 therefore the pickerel had commenced its existence as a species in the 

 two areas independently. But similar phenomena are of common 

 occurrence, though exceptional, as compared with the general facts 

 of distribution — and it did not seem that they justified the conclusion 

 drawn by M. Agassiz. It would be impossible to offer more than 

 suggestion towards explaining the particular case quoted, in a manner 

 reconcilable with the view that the species of pickerel had originally 

 proceeded from a common centre, inasmuch as many very important 

 data bearing on the ease were not at hand. He would therefore make 

 some general suggestions, and illustrate them by a parallel case, with 

 which he was more acquainted, being in fact that which had given 

 rise to this discussion. 



When it is said that species are distinct, nothing more is as a rule 

 really implied than that two series of forms shew such a degree of 

 difference that it is convenient to distinguish them by different names. 

 When the differences are small it is usual to call them varieties, but 

 at the present day the distinction between species and varieties can be 

 merely regarded as one of degree, and whether a new set of forms is 

 treated as a species or variety, depends partly on the habit of the 

 describer, partly on the amount of information he possesses as to 

 the existence of intermediate forms. 



The definition of Cuvier, which had long been accepted by naturalists, 

 that "A. species is a collection of individuals descended from one 

 another, or from common parents, and from those which resemble 

 them as much as they resemble themselves," is clearly of no use when 

 the question under discussion is whether two given distinct sets of 

 forms are, or may be, descended from a common stock. Actual 

 degrees of resemblance are in most cases the only criteria at the 

 command of a naturalist, and in a few cases the power of interbreeding 

 and producing fertile progeny. But the iifferences drawn from the 

 latter are by no means always in accordance with those drawn from 

 the former. The recent investigations of M. Ch. Naudin on the 

 hybridity of plants proved that in certain cases, species which in external 

 and anatomical characters were only distinguishable by great practice, 



4i 



