102 Contributions to Indian Malacology. [No. 2, 
238, &e., which calls for a few remarks. Although I differ in many 
points from Mr. Theobald’s views as put forward in this paper and in 
the earlier one of 1863, especially those on the origin, migration, and 
distribution of specific forms, I see no object to be attained in answer- 
ing at length opinions long since refuted, as I believe, by far more 
competent authorities and far abler writers. The works of Edward 
Forbes, Owen, Lyell and a host of others besides Darwin, will serve to 
shew the arguments relied upon by the great majority of living natur- 
alists, to prove the doctrine of “ specific centres,’ that is the theory 
that all members of the same species, whether existing or dead, have 
descended, not necessarily from one pair, but from one parent stock, 
living in one spot. To call this, however, the Darwinian theory, as 
Mr, Theobald appears to do, would be paralleled by calling the 
earth’s rotation round the sun the Newtonian theory. In each case 
the earlier theory is only a necessary step in the line of argument, and 
the hypothesis of the origin of species by means of Natural Selection 
is no more involved in the doctrine of specific centres, than was the 
theory of universal gravitation in that of the rotation of the planets 
around the sun. 
Ii I refer briefly to one remark of Mr. Theobald’s, (that in his first 
paper, J. A. S. B. for 1863, Vol. XXXII. p. 376) it is because it 
appears to me the only argument of any importance which he has — 
advanced in favour of his opinions. The question of the distribution 
of fresh water shells and especially of the bivalves, with their limited 
powers of progression, is a well worn argument in favor of the 
sporadic origin of species; that is, of the descent of each species from 
many parent stocks, existing in distinct and separate localities. But 
if all the facts of the case are fairly stated, there appears much, even 
in this instance, in favour of the doctrine of specific centres. The . 
facts are briefly these. Many species of Unio, e. g. U. marginalis, 
Lam. exist throughout a large tract of country, in almost every river 
and stream, and even in many ponds and marshes, although these 
rivers, &c. have no fresh water communication with each other what- 
ever, and the animal is incapable of living in the sea, or of traversing ’ 
the land. On the other hand, the area. inhabited by this species is 
continuous ; that is to say, the same species does not occur in tropical 
Asia and tropical America, for instance. Other species are restricted 

