FIVE GREAT ANIMAL GROUPS. 127 



nervous system, although formless, holding a certain position 

 in reference to the other organs. In Zoiiphyta, (figs. 57, 58,) it 

 is more collected, and arranged symmetrically, in reference to 

 right and left, and above and below. In Mollusca, (figs. 59, 60,) 

 it is more concentrated toward the side opposite the heart. In 

 Articulata, (figs. 61, 62,) the concentration is still further carried 

 out ; and finally in Vertebrata, (figs. 63, 64,) the nervous system 

 attains its highest confluence not only toward the median line 

 opposite the heart, but also in its tendency toward a head. 



Now it is a remarkable fact, that, as we trace the arrangement 

 of the systems of organs from the lowest to the highest groups, 

 we find the tendency is, in one sense, to carry out the idea of 

 polarity, which we see in the egg, to its strongest expression. 

 Thus, among the lower animals, the egg has the two poles (page 

 34, fig. 15) not distinct, since the opposing oil (ol) and albumen 

 (alb) merge more or less into each other; but among the higher, 

 (page 35, fig. 18,) the two poles, yolk (ol) and germ vesicle ( p) 

 are very marked. So it is in regard to the organs of animals ; 

 for the lowest of them, as I have shown, gradually differentiate 

 the opposing sides, the nervous and the digestive, until, as we 

 rise to the highest forms, we find the digestive system, or, as 

 the heart is a part of it, the nutritive centre corresponding to 

 the yolk, and the nervous centre corresponding to the albumen, 

 or germinal-vesicle pole. 



When we draw a line from one of these poles to the other, 

 whether we do it in the lowest or the highest animals, we divide 

 the body exactly into right and left ; that is to say, we find that 

 all animals are double, even man. 



The brain of man is double ; one half may be taken away 

 altogether, without affecting the mental functions, as certain 

 diseases have shown. The experiment of removing one half 

 of the brain has been tried successfully on dogs, rabbits, and 

 pigeons, and these animals did not lose their usual mental 

 powers. The paralysis of one side of a man's body, whilst the 

 other half retains its powers of motion, shows that the spinal 

 cord is also double, like the brain. The duplicity of this system 



