OBGANIZATION OF ANIMALS. 191 



348. But as we ascend in the scale, we find that this 

 simplicity of structure is confined to a very few species, and 

 that the functions become more and more localized and 

 specialized ; and, with this specialization, more and more 

 varied and perfect. 



A first degree in this localization of physiological phe- 

 nomena, is found in the earth-worm (lumbricus terrestris), 

 whose body offers a series of segments of identical parts ; that 

 is, in each segment we find a portion of the alimentary canal, 

 nervous system, and circulation; but these segments are 

 identical, or repetitions of each other, presenting no special 

 organ on which life in a peculiar way depends ; and thus, if 

 divided into five, ten, or twenty segments, each segment will 

 continue to live and become an independent being. 



But, for obvious reasons, this cannot be done with an 

 animal in which the organs have become so specialized 

 that on the integrity of some localized distinct segment, 

 not common to all, the general vitality of the being 

 depends. 



349. Already in insects we distinguish a more conside- 

 rable division of labour in respect of the organs ; the faculty 

 of perceiving certain sensations and of producing voluntary 

 motions, comes to be concentrated in certain nervous ganglions 

 lodged in the head; this concentration of functions goes on 

 increasing as we ascend, and embraces nearly the whole range 

 of the animal economy, ascending through the various 

 animals to man himself. Now it must be obvious that the- 

 destruction of one of these specialized organs, located in a 

 single segment of the body, and all-important to life, must 

 entail the destruction of the animal. 



350. Organic Transformations and Tendency to Uni- 

 formity of Composition. The complication in structure as 

 we ascend in the scale of being, takes place in some instances 

 by the creation of organs completely new, which are thus 

 superadded to those already existing in the lower animals ; 

 but more frequently the complication is effected in another, 

 and, as it would seem, a much more economic way. Thus, 

 in a great number of instances, the localization of the func- 

 tions is determined by a simple modification in the disposition 

 of parts already existing in the lower animals, a modification 

 by which these materials are adapted to the special purpose or 

 use, and not to a general one. In the Molucca crab (Limulus, 

 Fig. 142), the limbs of the cephalic and thoracic portions of the 



