'5 12 



while the cerebral hemispheres become wonderfully 

 complex and enlarged, bringing us to the highest devel- 

 opment, in man. 



The history of the circulatory system in the Verte- 

 brates is the same.* First, a heart with one chamber, 

 then one with two divisions : 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 arrange- 

 ment 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 re- 

 sulted 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, oblitera- 

 tion or subdivision. Of the former character are rudi- 

 mental 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 em- 

 bryology we can prove most indubitably that the simple 

 and less complex are inferior to the more complex. 



* See a homological system of the circulatory system in th author's Origin 

 of Genera, p. 32. 



