20 ORGANIC DEPENDENCE AND DISEASE 



The Proper Understanding of the Shell or External Skele- 

 ton. It is well recognized throughout the evolution of or- 

 ganic beings that a feature acquired as an advantage in the 

 fight for existence is easily carried beyond the point of ad- 

 vantage into a disqualification or obstacle in the same strug- 

 gle. The elephant's tusks, the narwhal's horn, the moose's 

 antlers, the sabre-toothed tiger's canines, bony collars and 

 dorsal crests in the ancient reptiles, stony spines on head 

 and body in infinite variety among invertebrates, are ready 

 representatives of this fact. It is specialization-develop- 

 ment carried from usefulness into disadvantage. The hard- 

 ening of the outside coat of primitive organisms or the cre- 

 ation of an external shell was, in its inception, a definite 

 protective advantage so adjusted by secretion that it could 

 not impair the activity of any function. In many of the sim- 

 pler expressions of life, the Radiolaria, the Foraminifera 

 and sponges, these mineral deposits were not permitted to 

 interfere with the easy movements of the protoplasmic or 

 simple cell contents, and so if the scattered mineral parti- 

 cles became united into a solid framework, there were defi- 

 nite openings and holes left for such movements. As net- 

 works of minute rods or stars or little burrs, or in other 

 forms of beauty and symmetry, built up by an unexplained 

 directive process, the mineral matter is often disseminated 

 through epithelial or epidermal walls, as in the sponges, or 

 compactly joined together into definite continuity, as in the 

 corals. The starfish and the crinoids have aggregated the 

 skin deposits about centers out of which growth has often 

 developed solid plates which press against one another 

 without uniting and so produce a covering with some de- 

 gree of elasticity. 



In the type of external skeleton shown by the mollusks, 

 to which we have referred, the clam, the snail, the nautilus 

 and their allies, the epithelium or mantle builds up by spic- 

 ular calcification a hard continuous covering which actually 

 embraces or is competent to embrace the entire animal ; an 



