Gradation of the Vertehraia. 145 



bill. Presence or absence of certain parts^ especially connecting 

 imrts. 



The corpus callosum, wliicli in the higher mammalia is the 

 largest and most important mass of commissural fibers, is want- 

 ing below that class and has a markedly varied development 

 within it. The pons Varolii and other commissures are want- 

 ing in many of the lower forms, the anterior commissure 

 being the only one traceable in fishes. The absence of these 

 parts indicates want of association in action and consequent 

 want of jjower to perform the higher complex functions, either 

 mental or physical. A single illustration from Dr. Carpenter : 

 " It is interesting to observe that in many Lepidoptera and 

 Hymenoptera, which are remarkable for rapid and powerful 

 flight, the nerves supplying both pairs of wings, are nnited at 

 their origins. On the other hand, in many insects which are 

 not remarkable for velocity or equability of motion, the nerves 

 supplying each wing originate separately, and have little com- 

 munication, just as in the larva of the Sphinx ; and in the Col- 

 eoptera, in which the upper pair or elytra are motionless dur- 

 ing flight, the nerves frequently remain entirely separate." 



Ahsence of commissural parts indicates low rank.. 



6th. Relative size (a) of ganglia of the same animal {h) of a 

 single ganglion to the lohole hody^ (c) of the encephalon and of the 

 entire nervous system to the whole hody — {d) of the parts of the 

 same ganglion., (e) of connecting parts. 



(a) It will not be questioned that the several ganglia per- 

 form different functions, that some of these functions are higher 

 and some lower, that size, with modifications, indicates their 

 functional power, that the cerebrum performs the highest func- 

 tions, that there is a scale of rank for the others, whatever it 

 may be. It is almost axiomatic then to state that the larger 

 the ganglia of high functions the higher the animal's rank. 

 The cerebrum in the lowest vertebrates is entirely wanting, in 

 all the lower orders it is relatively small, in the higher 



