620 — CHANGES IN THE CASPIAN. 
case of coast blockade by a foreign power. The difficulties of 
the undertaking are no doubt formidable, but the expense of 
maintenance and the uncertainty of the effects of currents set- 
ting through the new strait are still more serious objections.* 
Changes in the Caspian. 
The Russian Government has contemplated the establish- 
ment of a nearly direct water communication between the Cas- 
pian Sea and the Sea of Azoff, partly by natural and partly by 
artificial channels, and there are now navigable canals between 
* The opening of a channel across Cape Cod would have, though perhaps to 
a smaller extent, the same effects in interchanging the animal life of the 
southern and northern shores of the isthmus, as in the case of the Suez Canal; 
for although the breadth of Cape Cod does not anywhere exceed twenty miles, 
and is in some places reduced to one, it appears from the official reports on the 
Natural History of Massachusetts, that the population of the opposite waters 
differs widely in species. 
Not having the original documents at hand, I quote an extract from the 
Report on the Invertebrate Animals of Mass., given by Thoreau, Hacursions, p. 
69: ‘‘The distribution of the marine shells is well worthy of notice as a 
geological fact. Cape Cod, the right arm of the Commonwealth, reaches out 
into the ocean some fifty or sixty miles. It is nowhere many miles wide; but 
this narrow point of land has hitherto proved a barrier to the migration of 
many species of mollusca. Several genera and numerous species, which are 
separated by the intervention of only a few miles of land, are effectually pre- 
vented from mingling by the Cape, and do not pass from one side to the other. 
. . . Of the one hundred and ninety-seven marine species, eighty-three do 
not pass to the south shore, and fifty are not found on the north shore of the 
Cape.” 
Probably the distribution of the species of mollusks is affected by unknown 
local conditions, and therefore an open canal across the Cape might not make 
every species that inhabits the waters on one side common to those of the 
other; but there can be no doubt that there would be a considerable migration 
in both directions. 
The fact stated in the report may suggest an important caution in drawing 
conclusions upon the relative age of formations from the character of their 
fossils. Had a geological movement or movements upheaved to different levels 
the bottoms of waters thus separated by a narrow isthmus, and dislocated the 
connection between those bottoms, naturalists, in after ages, reasoning from 
the character of the fossil faunas, might have assigned them to different, and 
perhaps very widely distant, periods. 
