94 THE POPULAR SCIENCE MONTHLY. 



functions separately concentrated. Its aeration, its circulation, its 

 sensation must have single, localized centers, and its limbs be reduced 

 to the smallest possible number, and separated in duty. These requi- 

 sites are only fully attained in the human form. They are constitu- 

 tionally 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 par- 

 ticularly 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 becom- 

 ing 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 concen- 

 trated into one ganglion, while a single head-ganglion, of large size, 

 ofiiciates as a brain. Their muscular force is greater in projjortion 

 than that of Vertebrates, so that they are strong, active, and endur- 

 ing 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 develoi^ment. The three sections of the in- 

 sect body, each devoted to a single class of duties, has given them varie- 

 ty 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 pro- 

 duced 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 con- 

 stituted 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- 



