DEVELOPMENT. 



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 Rattle- 

 snake has but one lung, the second remaining in a rudi- 

 mentary condition ; both eyes of the adult Flounder and 

 Halibut are on the same side; the claws of the Lobster 

 differ ; and the valves of the Oyster are unequal. But all 

 these animals and their organs are perfectly symmetrical 

 in the embryo state. 



Again, animals exhibit a certain correspondence be- 

 tween the fore and hind parts. 181 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, theoretically per- 

 haps, be compared to two individuals placed side by side. 

 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 func- 

 tions are more dissimilar. An extreme want of sym- 

 metry 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, while 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 

 to say precisely what shall guide us in assorting living 



