DEVELOPMENT. 



223 



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 ; Rabbits have 

 been born with one ear, and Stags with one horn; the 

 Rattlesnake has but one lung; both eyes of the Flounder 

 and Halibut are on the same side; the claws of the Lob- 

 ster differ; and the valves of the 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. 121 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. So a Vertebrate may be considered not only as two 

 individuals placed side by side, but also as two individu- 

 als put end to end — the head and arms representing one, 

 and the legs the other. 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 ex- 

 treme w r ant of symmetry is seen in Birds which combine 

 aerial and land locomotion. 



There is also a tendency to a vertical symmetry, or 

 up-and-down arrangement — the part above a horizontal 

 plane being a reversed copy of the part below. A good 

 example is the posterior half of a Cod, w r hile the tail of a 

 Shark shows the want of it. This symmetry decreases as 

 we ascend the scale. In most animals there is consider- 

 able difference between the dorsal and ventral surfaces ; 

 and in all the nervous system is more symmetrically dis- 

 posed than the digestive. 



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 



