66 THE WHENCE AND THE WHITHER OF MAN 



perhaps, be considered as the foreshadowings of a 

 structure not yet realized ; they can only by courtesy 

 be called a head. Thirdly, the insect has legs. The 

 annelid had fin-like parapodia, approaching the legs of 

 insects about as closely as the fins of a fish approach 

 the legs of a mammal. The reproductive and digestive 

 systems, while somewhat improved, are not very 

 markedly higher than those of annelids. The ex- 

 cretory system has more work to perform and reaches 

 a rather higher development. 



But in these organs there is no great or striking 

 change ; the time for marked and rapid development 

 of the digestive and reproductive systems has gone by. 

 Material can be more profitably invested in brain or 

 muscle. Air is carried to all parts of the body by a 

 special system of air-sacks and tubes. This is a very 

 advantageous structure for small animals with an ex- 

 ternal skeleton. In very large animals, or where the 

 skeleton is internal, it would hardly be practicable ; the 

 risk of compression of the tubes at some point, and of 

 thus cutting off the air-supply of some portion of the 

 body, would be altogether too great. 



The circulatory system is very poor. It consists 

 practically only of a heart, which drives the blood in an 

 irregular circulation between the other organs of the 

 body much as with a syringe you might keep up a 

 system of currents in a bowl of water. But the rapid- 

 ity of the flow of the blood in our bodies is mainly to 

 furnish a supply of oxygen to the organs. A tea- 

 spoonful of blood can carry a fair amount of dissolved 

 solid nutriment like sugar, it can carry at each round 

 but a very little gas like oxygen. Hence the blood 

 must make its rounds rapidly, carrying but a little 



