98 MORPHOLOGICAL DEVELOPMENT. 



the direction of its length; but if the file has to oppose an 

 enemy or remove an obstacle lying in the line of its march, 

 the front man is the only one able to use his weapons or 

 hands to much purpose. And manifestly such an arrange- 

 ment could become advantageous only if the front man pos- 

 sessed powers peculiarly adapted to his position, while those 

 behind him facilitated his actions by carrying supplies, &c. 

 This simile, grotesque as it seems, serves to convey better 

 perhaps than any other could do, a clear idea of the relations 

 that must arise in a chain of individuals arising by gemma- 

 tion, and continuing permanently united end to end. Such 

 a chain can arise only on condition that combination is more 

 advantageous than separation; and for it to be more advan- 

 tageous, the anterior members of the series must become 

 adapted to functions facilitated by their positions, while the 

 posterior members become adapted to functions which their 

 positions permit. Hence, direct or indirect equilibration or 

 both, must tend continually to establish types in which the 

 connected individuals are more and more unlike one another, 

 at the same time that their several individualities are more 

 and more disguised by the integration consequent on their 

 mutual dependence. 



Such being the anticipations warranted by the general laws 

 of evolution, we have now to inquire whether there are any 

 animals which fulfil them. Very little search suffices; for 

 structures of the kind to be expected are abundant. In that 

 great division of the animal kingdom at one time called An- 

 nulosa, but now grouped into Annelida and Arthropoda, we 

 find a variety of types having the looked-for characters. Let 

 us contemplate some of them. 



205. An adult Chastopod is composed of segments which 

 repeat one another in their details as well as in their general 

 shapes. Dissecting one of the lower orders, such as is 

 shown in Fig. 160, proves that the successive segments, be- 

 sides having like locomotive appendages, like branchiae and 



