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 
tight 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 lias 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 
