MUSEUM OF COMPARATIVE ZOOLOGY. 181 
Sigsbee near Havana, but in over four hundred fathoms, quite a large 
number of common Cuban land shells were found, beside quantities of 
marsh grass, bits of rattan, bamboo, sugar-cane, dead leaves, etc., all of 
which were in good condition. If fossilized with the living sea shells 
dredged with them, the deposit, as observed by Prof. Agassiz (Bull, 
M. C. Z., Vol. V. p. 295), might sorely puzzle paleontologists of a future 
century seeking to determine the circumstances under which it was 
formed. 
When we consider the great uniformity of texture of the deposits 
forming the floor of the oceanic deeps, it would seem as if the envi- 
ronment offered attractions for only a limited variety of forms. The 
bottom is generally composed of extremely fine impalpable mud, and in 
many portions of the abyssal area offers no stones or rugose inorganic 
objects for sedentary mollusks to perch upon. It is not quite destitute 
of such stations, however, and all are utilized by the abyssal popula- 
tion. In the absence of stones, many unusual selections are made. 
The chitinous tubes of hydroids and the irregular leathery dwellings 
of tubicolous annelids are occupied, after their original owners are dead 
or dispossessed, by various little limpets, such as Lepetella and Cocculina, 
The long spines of the abyssal sea-urchins or echini offer a welcome 
perch for species of Capulus, which, when they grow too large to find 
a satisfactory foothold, secrete a shelly pedestal which serves them for 
life. The carbonic acid in the water rapidly destroys the shells of such 
mollusks as die in the great depths, so that they do not form gravelly 
accumulations or “coquina” rock, as in shallower waters. A bivalve, 
Modiola polita, related to the ordinary mussel of N orthern seas, spins a 
sort of nest of stout byssal threads, in which it is completely concealed, 
and which protects in its meshes, not only the young fry of the maker, 
but various little commensal animals of different orders, such as mol- 
lusks, worms, and crustacea. 
In the evolution of animal life two classes may be recognized: those 
which maintain successfully the struggle for existence by facility in vary- 
ing their superficial characters to meet the exigencies of their environ- 
ment, — in short, by their facile plasticity ; and a smaller group, which 
seem to have an innate strength of constitution which resists the influ- 
ence of changes in the environment better by a dogged persistence in 
their original form. These respond little, if at all, by external variation, 
to the ordinary fluctuations of the physical world about them. This 
has been noted by Darwin in birds, in his comparison between the vari- 
ations of pigeons and the “inflexible organization” of the goose. But 
