Chap. XVI. ] 



SOME OTHER MAMMALS, 



259 



It must not be supposed that anything like a regular 

 serial order or progression is to be observed in the 

 development of the Brain among Mammals. In the 

 higher types of lower orders it will often be found 

 better developed than among the lower types of higher 

 orders. Still if we compare the extremes of the class — • 

 that is, higher with lower Mammals — a great increase in 

 the developmental complexity of the organ, or in type of 

 Brain, as judged by the human standard, will become 

 perfectly obvious. 



The ratio of the weight of the Brain to the weight of 

 the body, is subject to great variations, from different 

 causes, so that a table of such ratios does not give any 

 trustworthy information as to the relative development of 

 the organ in different species of animals. We may be 

 able to deduce some kind of rough average, sufficing to 

 indicate its increasing development if we compare class 

 with class — as Fishes with Birds, or Birds with Mammals 

 — but in detail and for estimating the relative develop- 

 ment of the Brain in different species, its indications are 

 of little or no value. This may be illustrated by the 

 following table in which some of these ratios are given : — 



in the above series is one which does not correspond with 



