MAN A VERTEBRATE 9 



type of frame. There are minor modifications to 

 suit the various kinds of environment, adaptations 

 to the necessities of aquatic, terrestrial, and aerial 

 locomotion and life, some parts being specialised, 

 others atrophied, and still others omitted, but 

 there is never anywhere, from fishes to philoso- 

 phers, any fundamental departure from the estab- 

 lished vertebrate type of skeleton.* The pectoral 

 fins of fishes correspond to the fore-limbs of frogs 

 and reptiles, the wings of birds, and the arms of 

 men. The pelvic fins of fishes are homologous 

 with the hind-limbs of frogs, reptiles, and quad- 

 rupeds, and the legs of birds, apes, and men. 

 The foot of the dog and crocodile, the hand of the 

 orang, and the flipper of the dolphin and seal, all 

 have the same general structure as the hand of 

 man ; and the wings of the bat and bird, the fore- 

 limbs of the lizard and elephant, and the comical 

 shovels of the mole and ornithorhynchus, notwith- 

 standing the great differences in their external 

 appearance and use, contain essentially the same 

 bones and the same arrangement of the bones as 

 do the arms of men and women. The human 

 body has two primary cavities in it. So have the 

 bodies of all vertebrates : a neural cavity con- 

 taining the brain and spinal cord, and a visceral 

 cavity containing the heart, liver, lungs, and 

 alimentary canal. Invertebrates have only one 



* Snakes are limbless, and hind-limbs are lacking in 

 whales and other degenerates ; but rudimentary limbs are 

 found in the embryonic stages of these animals. Frogs, it 

 may be said also, have no ribs. 



