APPARENTLY CORRELATED TO THE (JONDPTIONS OF LIFE. 
:317 
terrace these several attributes would be found in varying degree among the individual 
shells—that some would be thick and some thin, some long and some short, &c. ; on 
the other hand, if the new qualities were the result of the new conditions, then it 
would be anticipated that the shells of each terrace would be all nearly similar in 
texture and shape. The more uniformly any of the new variations are found among 
the individuals, the more probable is it that they are due to the direct action of 
environmental change rather than to natural selection ; but a new quality, wliich is 
found in the several individuals to a greatly varying degree, cannot be held to be 
shown to be the direct result of the conditions, even though it be found to be 
increasingly more marked on the average in successive generations as the conditions 
to which it is supposed to be due become more intense. Now the variations formed 
among these shells are of two kinds. The variation in proportional length, though 
becoming more and more marked in the shells which have been exposed to salter 
water, is not found in all the individuals {vide Tables) ; on the other hand, the 
variations in the quality, texture, and colour of the shell are found developed to 
nearly the same degree among all the individuals of successive terraces. Hence it 
may be fairly supposed, in the case of these latte]’ variations, that they are really due 
to direct environmental change. The same also is true of the shells from the fresh¬ 
water localities, the texture and colour of which are practically uniform, while a good 
deal of variation is found in the shape, though the general prevalence of the long type 
is clear. In view of the manifest connection between variation in the texture, &c., of 
the shells and the conditions in the lake, it would be interesting to know more clearly 
the mode of action of these conditions in producing those effects, but as to this it is 
difficult to make a conjecture. No doubt they are the result of changes in the nutri¬ 
tion of the animals, but more than this does not seem clear. It can scarcely be 
supposed that the thinness of the shells was due to a deficiency of lime in solution in 
the water, since this would rather increase in relative amount with the evaporation. 
Moreover, the shells from the two fresh-water lakes at Ramleh are fairly thick. 
Neither can the deficiency in the amount of the shells be due to general starvation, 
since there is no diminution in absolute size at Shumish Kul, except in the case of the 
shells of the lowest level, which.do appear generally ill-nourished; while, at Jaksi 
Klich, those shells which have become thin and papery from the desiccation of the 
lake are, on the whole, absolutely larger than an average sample of shells from the 
Aral Sea. 
It may here be remarked that the striking similarity between the shells wliich had 
been exposed to very salt water at x4bu Kn and tlmse of the salt lakes of the Aral 
region has features of special interest, since not only have the similar conditions 
prevailed in producing two forms closely resembling each other, but this has been 
achieved, though the animals subjected to the influence Avere at first unlike and had 
had a very different history. For even supposing that the Cockles in the Aral Sea 
were originally derived from those of the Mediterranean, which is uncertain, yet the 
