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Page:Popular Science Monthly Volume 18.djvu/104

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THE POPULAR SCIENCE MONTHLY.

functions separately concentrated. Its aƫration, its circulation, its sensation must have single, localized centers, and its limbs be reduced to the smallest possible number, and separated in duty. These requisites are only fully attained in the human form. They are constitutionally prohibited to the segmented animals.

In the insects the persistent individuality of the segments is shown partly in their six legs, each pair attached to a segment; but more particularly in their generalized nervous and respiratory systems. To a great extent each segment preserves its nerve-ganglion. So, to a similar extent, each segment does its own breathing, the whole body becoming one generalized lung. The blood circulation, which is only partly confined to specialized blood-vessels, is accompanied by a general air circulation. There is nothing resembling the localized relations of these circulating systems as seen in the Vertebrates.

Such are the constitutional limitations to development in the Articulates, probably resulting from their social origin. The effort to overcome these limitations in the crabs has resulted in organic conditions opposed to a high development. How is it in the insects? In them the segments are so welded as to form three distinct body sections. In the higher insects the individuality of the segments is so reduced that the nerve-ganglia of the thoracic segments are concentrated into one ganglion, while a single head-ganglion, of large size, officiates as a brain. Their muscular force is greater in proportion than that of Vertebrates, so that they are strong, active, and enduring in bodily vigor. What natural influence is it that has restricted their development?

This may not be difficult to discover. We have seen that the too great compacting of the articulated body, as in the crabs and spiders, has proved a hindrance to development. The three sections of the insect body, each devoted to a single class of duties, has given them variety of motion, and more diversified food-getting functions. But it has otherwise worked injuriously. Rapid variations in movement require that these sections should be united by flexible joints. But these joints are articulations of an external skeleton, and can only be produced by a deep depression of this cortical armor into the regions dividing the sections. Thus the continuity of the body is almost broken at these joints. A similar relation exists between the joints of the limbs.

It seems evident from these considerations that the insect is not constituted to attain a large size. Conditions which are suitable to a small body might prove utterly unfitted to the requirements of a larger organism. Let us imagine an insect of the size of an ox; walking on its six many-jointed, hollow legs; its body composed of three almost separate sections; breathing through air-holes in its sides, its whole body but an air-tank, or lung. Even if such a growth were possible, it would obviously be at a disadvantage as compared with the Verte-