438 ON CONCRETIONS, SPICULES, [ch. 



if we try to make a rough classification, by way of forecast, of 

 the chief conditions which we are Ukely to meet with. 



Just as we look upon animals as constituted, some of a vast 

 number of cells, and others of a single cell or of a very few, and 

 just as the shape of the former has no longer a visible relation to 

 the individual shapes of its constituent cells, while in the latter 

 it is cell-form which dominates or is actually equivalent to the 

 form of the organism, so shall we find it to be, with more or less 

 exact analogy, in the case of the skeleton. For example, our own 

 skeleton consists of bones, in the formation of each of which a 

 vast number of minute living cellular elements are necessarily 

 concerned ; but the form and even the arrangement of these 

 bone-forming cells or corpuscles are monotonously simple, and we 

 cannot find in these a physical explanation of the outward and 

 visible configuration of the bone. It is as part of a far larger 

 field of force, — in which we must consider gravity, the action of 

 various muscles, the compressions, tensions and bending moments 

 due to variously distributed loads, the whole interaction of a very 

 complex mechanical system, — that we must explain (if we are to 

 explain at all) the configuration of a bone. 



In contrast to these massive skeletons, or constituents of a 

 skeleton, we have other skeletal elements whose whole magnitude, 

 or whose magnitude in some dimension or another, is commensurate 

 with the magnitude of a single living cell, or (as comes to very 

 much the same thing) is comparable to the range of action of the 

 molecular forces. Such is the case with the ordinary spicules of 

 a sponge, with the delicate skeleton of a Radiolarian, or with the 

 denser and robuster shells of the Foraminifera. The effect of 

 scale, then, of which we had so much to say in our introductory 

 chapter on Magnitude, is bound to be apparent in the study of 

 skeletal fabrics, and to lead to essential differences between the 

 big and the little, the massive and the minute, in regard to their 

 controlling forces and their resultant forms. And if all this be 

 so, and if the raijge of action of the molecular forces be in truth 

 the important and fundamental thing, then we may somewhat 

 extend our statement of the case, and include in it not only 

 association with the living cellular elements of the body, but also 

 association with any bubbles, drops, vacuoles or vesicles which 



