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 m\xd, 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 Cocculiiia. 

 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 Northern seas, spins a 

 sort of nest of stout bygsal 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 



