ACCOMMODATION TO PKESSUEE. 327 



in Ireland, one of thousands, both larger and smaller, wliich 

 were pierced in the same way by myriads of these small boring 

 animals. Unfortunately it was impossible to examine the 

 animal very minutely at the time, and I could only ascertain 

 that it certainly was the Limnoria, so familiar to me on the shores 

 of Heligoland and elsewhere, which in Ireland had not disdained 

 the hardest limestone. Very likely closer investigation might 

 reveal certain differences in the different individuals of the same 

 species boring in wood and in stone, as to the structure of the 

 organs used in boring, and flte^ might show an evident relation 

 between those organs and the hardness of the substance bored 

 into. It is known, too, that certain species of sea-urchin some- 

 times bore into very hard rocks, while in other places they do 

 not. In other instances the form of the animal shows the 

 direct effects of contact with solid bodies; this is the case 

 with many sedentary animals or those enclosed in a solid shell. 

 Many oysters, and such shell-fish as establish themselves on 

 rocks or wood or in fissures, frequently adapt their shells very 

 exactly to their position, and the form thus given to them might 

 easily become constant, and even a fixed specific character, if a 

 similar position were adopted by all the individuals of the species. 

 But here again, we do rot know how far the determining 

 influence of the solid object, which in some measure moulds the 

 shell, may extend, for no experiments exist which can conclu- 

 sively prove that in such cases the moulding power of the posi- 

 tion has been exclusively effective with no assistance from 

 inheritance, and we therefore are not in a position to assert 

 that this explanation of the observed cases, though in itself 

 extremely plausible, is in every respect the right one. In order 

 to verify this usual and apparently coiTect interpretation of 

 these forms of shells, it would be necessary to prove by experi- 

 ment that species which had hitherto lived in some particular 

 situation lost the form thus impressed on their shells as soon as 

 their young were compelled to establish themselves in a position 

 different in character ; and, yet more, that they could be made 

 regularly to assume quite different shapes according to the 

 differences in their new habitat. Experiments of this kind that 

 have been made with oysters undoubtedly prove that a certain 



