1 6 INSECTS. 



produced by variation in the details of parts. In this 

 class are included man, beasts, birds, reptiles, and 

 fishes. 



It is now time to support the assertion that a tadpole 

 comes nearer in its nature to a horse, or an eagle, or a 

 mullet, or a man, than to the grub of a water-beetle born 

 and bred in the same stagnant pool with itself ; and to 

 do this a few words must be given first to those verte- 

 brate animals, and next to that section of the Invertebrata 

 to which insects belong. 



The tadpole is a young and undeveloped frog, and 

 can, of course, be spoken of only as a frog. Now the 

 frog, the horse, the bird, the fish, the man, agree in these 

 respects, they all possess an internal bony and jointed 

 framework, or skeleton, composed of living tissues, 

 nourished throughout life by bloodvessels, and growing 

 with the growth of the animal. To this framework the 

 muscles are attached. The principal parts of this 

 skeleton are the spinal column or backbone, with the 

 ribs, the skull, and the bones of four limbs. All these 

 parts are not however found in all vertebrate animals, and 

 indeed the backbone seems to be the only part of the 

 skeleton which is never wanting. Thus, for instance, the 

 frog has no ribs, the snake has no limbs, and there is a 

 fish which has no skull. Again, these parts, when 

 present, are in various animals variously modified, 

 enabling each to fill its own place in creation ; and in an 

 examination of these modifications we perceive the con- 

 nexion existing among these animals under the greatest 

 diversity of form and habit. 



To take the four limbs, for example. In man they 

 form two legs and two arms ; in the ape four arms ; in 

 the horse four legs. In the tortoise we might hesitate 



