DEVELOPMENT 



435 



There are many exceptions : the heart and liver of the 

 higher vertebrates are eccentric ; the nervous system of 

 mollusks is scattered; the hemispheres of the human 

 brain are sometimes unequal ; the corresponding bones 

 in the right and left arms are not precisely the same 

 length and weight ; the narwhal has an immense tusk on 

 the left side, with none to speak of on the other ; the 

 rattlesnake has but one lung, the second remaining in a 

 rudimentary condition ; both eyes of the adult flounder 

 and halibut are on the same side ; the claws of the lob- 

 ster differ; and the valves of an oyster are unequal. 

 But all these animals and their organs are perfectly sym- 

 metrical in the embryo state. 



Again, animals exhibit a certain correspondence be- 

 tween the fore and hind parts. 17 * Thus, the two ends 

 of the centipede repeat each other. Indeed, in some 

 worms, the eyes are developed in the last segment as 

 well as the first. In the embryo of quadrupeds, the 

 four limbs are closely alike. But in the adult, the fore 

 and hind limbs differ more than the right and left limbs, 

 because the functions are more dissimilar. An extreme 

 want of symmetry is seen in birds, which combine aerial 

 and land locomotion. 



Every animal is perfect in its kind and in its place. 

 Yet we recognize a gradation of life. Some animals are 

 manifestly superior to some others. But it is not so 

 easy to say precisely what shall guide us in assorting 

 living forma into high and low. Shall we make structure 

 the criterion of rank? Plainly the simple jellyfish is 

 beneath complicated man. The intricate and finished 

 build of the horse elevates him immeasurably above the 

 stupid snail. The repetition of similar parts, as in the 

 worm, is a sign of low life. So also a prolonged poste- 

 rior is a mark of inferiority, as the lobsters are lower 

 than the crabs, snakes than lizards, monkeys than apes. 



