136-(106) GRADATION I1SF ANIMAL LIFE. 



outward appearance, reveals a more specialized structure 

 which at once assigns him a place among the higher 

 worms or molluscs. 



Specialization, however, may be carried to such an ex- 

 cess as to degrade, instead of conferring higher rank. 

 Thus when it goes so far as to produce what is called an 

 irrelative multiplication of similar parts, that is, a mul- 

 tiplication of parts having a common purpose, but not 

 mutually dependent, it is a degrading process. Among 

 land-articulates, true insects have but seventeen or eigh- 

 teen body-segments in three main divisions and only six 

 legs, while spiders have never more than twelve segments 

 in two body divisions, and only eight legs. The ease 

 and graceful rapidity with which insects and spiders 

 move, show that these provisions are numerically quite 

 sufficient to secure the highest results in locomotion. 



Now there is another sub-class of articulates, the centi- 

 pedes, which have from one hundred to two hundred 

 body-segments mostly alike, and about as many pairs of 

 feet, or from twenty to nearly four hundred feet. These 

 creatures are, by so much, inferior to true insects and 

 spiders. It is interesting to note the fact that this de- 

 grading repetition of limbs does not promote locomotion. 

 The- common centipede with thirty feet, is, at his very 

 best, a slow traveller in comparison with an average 

 eight-legged spider ; while the iulus, the common cylin- 

 drical centipede, with over one hundred legs, is one of 

 the very slowest of moving creatures. 



I think that I am justified in adding the remark that 

 specialization is an elevating process only so far as it 

 works symmetrically on the various functions of the 

 system. But when it develops certain organisms far be- 

 yond the rest so as to destroy the symmetry of the de- 

 velopment, its tendency is to that degree to lower the 

 rank. A remarkable instance of this is the sword of 

 defence of the narwhal, an enormously extended spec- 

 ialization of a single tooth, the left incisor, while the 

 other teeth are nearly, or in the female, entirely, sup- 

 pressed. 



In this connection may also be mentioned the occur- 

 rence of organs in some unusual position, or to state it 

 in another way, the adaptation of some part of the body 

 by the presence of structure not usually fou.n4 ip.such a 



