554 E. A. MINCfllN. 



sclerites should not at their first appearance have been similar 

 in form to any other cell granules ; that is to say^ simply little 

 lumps of rounded or irregular, or perhaps of crystalline form. 



Whatever may have been the course of evolution, the sclerites 

 must at first have been extremely minute, and no sooner did they 

 appear than there was at once an opening, so to speak, for the 

 action of adaptive and selective forces, related to the life of the 

 organism as a whole, as apart from the molecular or mechanical 

 forces acting upon the sclerite itself. Dreyer's hypothetical 

 tetraxon sclerite, if it were really the fundamental spicular 

 form, could only be explained, it seems to me, in this way. 

 Natural selection, we will admit and proclaim, is not an 

 " internally formative " principle ; it could not have caused 

 the first appearance of sclerites, which must be attributed 

 primarily to variations in the cell metabolism. But natural 

 selection is, we are told, an " externally regulative " force ; 

 and given a simple sclerite of whatever form, there is imme- 

 diately something upon which external regulation can be exer- 

 cised ; but there is no reason to postulate that the primitive 

 sclerite should be of the tetraxon rather than of any other 

 type. 



In the foregoing discussion of Dreyer's theory I have 

 endeavoured to follow up his conclusions entirely from his own 

 premises ; and in order not to confuse the argument, I have 

 purposely refrained from mentioning one constant accompani- 

 ment of the spicules of calcareous sponges which seems to me 

 to make Dreyer's theory altogether inapplicable to that group 

 at least. I refer to the sjiicule sheath, which is nothing less 

 than an envelope of structureless organic material, inserted 

 between the spicule and its secreting cell. 



All the authors who have expressed any opinion as to the 

 spicule sheath, namely, Haeckel, Schulze, and Dendy (see 

 above), agree in regarding it as a layer derived from the 

 general gelatinous ground substance condensed round the 

 spicule. But since the sheath appears between the spicule 

 and its secreting cell, it obviously cannot be derived from the 

 ground substance outside the cell. It may, however, be an 



