316 
MR. W. BATESON ON SOME VARIATIONS OF CARDIUM EDULE 
Conclusion. 
The importance of these observations lies in the fact that, by examining and com¬ 
paring the shells, an opportunity is given of observing the origin of a set of structural 
variations in correlation to, and perhaps in consequence of, environmental changes 
which are to some extent ascertained. The first point which is to be noticed is that 
the shells of each sample, Avhether it be from a separate lake or only from a particular 
terrace, are more like to each other than to the shells of one of the other lakes, or to 
those of another terrace in the same lake as at Sbumish Kul, Avhere the shells of each 
terrace have a distinct appearance and character of their own, and may easily be 
known from the shells of higher or lower terraces. 
o 
The next feature of importance is the fact that, in the four independent cases— 
Sluunish Kul, Jaksi Klich, Jainan Klich, and the Egyptian lagoon Abu Kir'"—the 
shells which have lived under similar conditions, i.e., in very salt Avater, have become 
like each other, having the characters of thinness, high colour, small beaks, ribbing on 
the inside, and great relative length. In Anew of these four instances of similar 
variations occurring under similar conditions, it seems almost certain that these 
conditions are in some Avay the cause of the variations. Similarly, in the case of the 
two groups of Cockles from Mareotis which have been isolated and exposed to fresh 
water in separate lakes, the result has been to produce a form of shell in both cases 
Avhich is practically the same. Cases of this kind, in Avhich it is possible to observe 
the appearance and progress of a variation through successive generations in the same 
place, are so rare that it has seemed worth while to describe these shells in detail. 
The mode of occurrence of the shells in terraces at Shuniish Kul provides an almost 
unique opportunity for beholding the gradual succession of these changes. If, then, 
it is admitted that the structural changes in the shells are to be regarded as the 
consequence of the environmental changes in the water of the lake, the question 
arises to Avhat extent these structural changes follow directly on the changed circum¬ 
stances, and how far they may not be due to the natural selection of a different type 
as the fittest to live in the altered state. 
Now, while the cases given above do not give a definite answer to this question, 
they nevertheless contribute something towards it. The chief qualities which appear 
in the shells Avhich have been exposed to the increased saltness are comparative thin¬ 
ness, high colour, and increased length, together Avith diminished beaks. If, then, it 
is supposed tliat shells having these qualities Avere being gradually chosen by natural 
selection as the fittest for the new conditions, it would be expected that in each 
* Amongst the shells in the Cambridge University Museum collected by MacAndrew are a few 
Cockles from a lagoon at Tunis, which show the same features. Though, in the absence of fui’ther 
information as to the locality from which they were brought, nothing can be positively stated, yet it is 
likely that they afford another instance of a similar variation occurring under .similar conditions. 
