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; 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. 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, 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 
