120 Memoirs of the Indian Museum. [Vor. VI, 
Globosa and comorinensis are younger varieties and have not become so stabilized 
as fusus owing to non-isolation from the main mass of the acuta species. 
The peculiar geographical distribution of obtusa and acuta (including also globosa 
and comorinensis under the latter) is again most illuminative on the influence of differ- 
ences in environment upon separated groups of an originally united race in producing 
and stabilizing variations from the original stock. ‘The geographical distribution of 
these two principal varieties as shown in the sketch plan on Plate XII will make 
this clear. | 
As is shown, the barrier formed by Rameswaram and Mannar Islands and Adam s 
Bridge very nearly forms the dividing line, obtusa being found entirely north of this 
line while acuta monopolizes the whole coastal waters of the Gulf of Mannar. But in 
addition the latter is seen to pass some distance beyond the barrier named and so to 
invade the territory of obtusa. 
The hypothesis I present is this. We have incontestable evidence, chiefly fur- 
nished by the existing distribution of certain animals and plants in Ceylon and India, 
_ that the geological phase existing in the Gulf of Mannar and Palk Bay region antece- 
dent to the present condition was that of a land barrier stretching continuously from 
India to Ceylon in the region now known as Adam’s Bridge. 
Further we have geological evidence that this phase was preceded by one where 
the level was even lower than it is now and when no land whatever existed between 
what is now the Gulf of Mannar and Palk Bay. During this phase of absolutely free 
communication from Cape Comorin to Madras, I think we have reason to believe that 
a single form of Turbinella pirum peopled the whole stretch of these coastal waters. 
The conditions would, so far as we can see, be little divergent anywhere along such a 
coastline and there would be no special localized conditions adequate to stimulate the 
evolution of well-marked varieties. Such uniform conditions disappeared immediately 
upon the formation of a continuous land barrier between India and Ceylon on the line 
Pamban—Rameswaram—Adam’s Bridge--Mannar. Two isolated groups of the species 
would then be entailed; even connection by way of the east and south of Ceylon 
would be ineffective to keep up a connection between the isolated groups, because 
very deep water comes close to the land on parts of the E. and S.E. of Ceylon, and 
such deep water is as great a barrier to the dispersal of Turbinella as a land barrier, 
seeing that its larvae do not pass through a free-swimming stage. What we have seen 
happened in the case of the Andaman Turbinella happened again here. The shells in 
one or may be in both the localities cut off by the Indo-Ceylon barrier diverged from 
the common ancestor, and to-day we have two forms so strongly marked as to con- 
stitute distinct varieties. Had the land barrier not been broken down, these two 
varieties would assuredly have hardened into distinct species. With the breaking 
down of the barrier before this was effected, we get a mingling of the two main 
varieties and the possibility of the eventual suppression of one of them. Of this, I 
am, however, doubtful; I believe rather that the obtusa form has evolved characters 
fitting it to contend with certain conditions peculiar to Palk Bay and neighbourhood 
—of which muddy water is one—better than the acuta variety, which is more adapted 
