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1883.] Growth and Development. 721 
for safety. Another form of armor may begin in the chance 
clinging of sand to the jelly-like body. All such favored forms 
are sharply selected from the multitude of variations, and thus 
assume the definiteness of species. The intermediate, weakly- 
protected forms are crowded out. 
The subsequent evolution of naked and armored forms must 
necessarily differ. The one becomes generally sensitive, gains 
varied motor organs, and becomes swift and diverse in its powers 
of motion. The other is sluggish and lacks sensitiveness. Sensation 
is confined to the unprotected parts, and it is these which de- 
velop into elongated organs of touch and movement. In the 
one food is obtained by swift approach, safety by swift retreat. In 
the other, food is usually obtained through currents made by cilia 
or tentacles in the water, safety by a withdrawal within the 
armor. 
In all animals above the very lowest it is of importance that 
the surfacé should grow in some degree indurated. If the naked 
protoplasm were exposed freely to every contact there would be 
constant motion in response, and the energies of the body be 
uselessly and dangerously exhausted. The animals best adapted 
are those which have limited and partly protected parts of the 
surface alone exposed to the influence of the finer modes of force 
contact, while the protoplasm of the remainder of the surface is 
sensitive only to the more vigorous impacts. j 
Surface induration may take one of two forms. It may in- 
crease until the skin becomes a hard armor, to which the animal 
trusts more than to motion, it becoming heavy in weight, slow in 
movement and dull in sensation. Or it may end at a slight de- 
gree of induration, the animal being light in weight, quick in 
movement and sharply sensitive. Thus the two phases of evolu- 
tion which appear in the lowest animals, reappear in the higher 
With similar results. E 
All protoplasm is sensitive to touch of all kinds, when exposed 
to it, but each separate kind of touch tends to develop conditions 
of appropriate sensitiveness. Excessive light causes a general 
development of dark pigment, probably as protective against heat 
effects through its active radiation. This aids absorption of the 
light rays, and is the condition preliminary to the evolution of the 
eye. Sound also tends to develop a receptive organ, The pre- 
liminary condition of this organ is the deposition of solid parti- 
