134 GENERAL EVOLUTION. 



subdivided, but scarcely according to the complex type that fol- 

 lows. In fishes the cerebellum and cerebral hemispheres are 

 minute, and the intermediate or optic lobes very large : in the 

 reptiles the cerebral hemispheres exceed the optic lobes, while the 

 cerebellum is smaller. In birds the cerebellum becomes complex 

 and the cerebrum greatly increases. In mammals the cerebellum 

 increases in complexity or number of parts, the optic lobes 

 diminish, while the cerebral hemispheres become wonderfully 

 complex and enlarged, bringing us to the highest development, 

 in man. 



The history of the circulatory system in the Vertebrates is the 

 same. First, a heart with one chamber, then one with two divis- 

 ions : three divisions belong to a large series, and the highest 

 possess four. The origins of the great artery of the body, the 

 aorta, are first five on each side : they lose one in the succeeding 

 class in the ascending scale, and one in each succeeding class or 

 order, till the Mammalia, including man, present us, with but one 

 on one side. 



From an infinitude of such considerations as the above, we 

 derive the certainty that the general arrangement of the various 

 groups of the organic world is in scales, the subordinate within 

 the more comprehensive divisions. The identification of all the 

 parts in such a complexity of organism as the highest animals 

 present is a matter requiring much care and attention, and con- 

 'stitutes the study of homologies. Its pursuit has resulted in the 

 demonstration that every individual of every species of a given 

 branch of the animal kingdom is composed of elements common 

 to all, and that the differences which are so radical in the higher 

 groups are but the modifications of the same elemental parts, 

 representing completeness or incompleteness, obliteration or sub- 

 division. Of the former character are rudimental organs, of 

 which almost every species possesses an example in some part of 

 ^its structure. 



But we have other and still more satisfactory evidence of the 

 meaning of these relations. By the study of embryology we can 

 prove most indubitably that the simple and less complex are in- 

 ferior to the more complex. Selecting the Vertebrates again as 

 an example, the highest form of mammal — e. g., man — presents 

 in his earliest stages of embryonic growth a skeleton of cartilage, 

 like that of the lamprey ; he also possesses five origins of the 

 aorta and five slits on the neck : both which characters belong to 



