134- (104) GEADATIOTsT I1ST ANIMAL LIFE. 



zation (a term derived from the Greek word meaning 

 head), implies the concentration of nervous power, at, 

 or near, the head. Many small animals of very low grade 

 fail to exhibit any nervous organization. 



The presence of nerve-matter in one or more small 

 masses called "ganglia," from which nerve cords stretch 

 to various parts of the body, as in molluscs, is a higher 

 form ; yet if these ganglia are nearly of a size and dis- 

 tributed over the organism, the rank is still very low. 



There is a decided step upwards, and towards cephali- 

 zation, when there is a head-ganglion, larger than the 

 others. A still higher grade is reached when the nerve 

 matter forms a cord running lengthwise through the 

 body. Yet the relative x>ositioh of this nervous cord has 

 also an important bearing on relative rank. In insects, 

 where this cord courses the ventral or lower parts of the 

 body, the rank is lower than in vertebrates where it 

 courses along the upper part or back. 



Still another upward movement occurs in those animals 

 whose nervous dorsal cord has a large expansion at the 

 anterior end, forming a distinct head. If the cord, how- 

 ever, stretches far to the rear, strong and stout, energiz- 

 ing posteriorly many and important organs, as compar- 

 ed with those about the head, and especially if it extend 

 into a long or powerful tail, in so far, the grade is very 

 low for a vertebrate. But in proportion to the relative 

 increase in capacity and compactness in the brain, en- 

 abling it to energize the most numerous and important 

 organs in subsidy to the wants of the head, the grade of 

 the vertebrate is high. Here, as we take one last step 

 higher, the question of relative position again comes in. 

 It has been remarked by Prof. Dana that in the higher 

 vertebrates the sheathed nerve-column, or vertebra, as- 

 sumes a more and more erect position, indicative of rise of 

 grade, until in man, who alone of animals can stand erect, 

 the brain is in a vertical line above the entire nervous sys- 

 tem ; it demoninates the whole, and the summit of rank 

 is reached. Under the application of this law of cephali- 

 zation, shrimps and lobsters whose abdomens are prolong- 

 ed in a long robust tail-lijve fashion, are inferior to crabs, 

 for the crabs have a very short and insignificant abdo- 

 minal tail, tucked up almost out of sight, under the 

 broad cephalothorax, as if they recognized the disgrace 



