Like?iesses ; or, Philosophical Anatomy 257 



might be simply described as parts which resembled each 

 other, because they were alike descended from one single 

 part in a remote common ancestor. 



Soon, however, investigation rendered necessary farther 

 analysis, with respect to parts said to be homologous. It 

 came to be recognised that there are likenesses between 

 different animals and different parts of the same animal, 

 which a theory of common descent cannot explain; and 

 'similarity in relative position' had to be once more had 

 recourse to as a definition of what was meant by homology, 

 such similarity being, in certain cases, explicable by 'descent,' 

 and in others not so explicable. 



A very obvious example of likeness not explicable by 

 ' descent ' is the familiar one referred to in the early part 

 of this paper, between our right hand and our left. This 

 Ukeness is part of that general correspondence which exists 

 between the right and left sides of most animals, and which 

 is spoken of as 'bilateral symmetry,' or lateral homiology. 

 Another example is that likeness which sometimes exists 

 between parts placed one above another, as between the 

 upper and lower parts of the tail-fin of most fishes. Such 

 likeness is an example of 'vertical symmetry,' or vertical 

 homology. Another kind of 'likeness,' or homology, is 

 termed ' serial.' It is chiefly in our hmbs that this kind 

 of homology is manifested externally in us, but it is plainly 

 enough to be seen in the human skeleton (or in that of any 

 backboned animal), in the ribs or in that series of generally 

 similar bones (vertebrae) which make up our vertebral 

 column or backbone. Our limbs, however, as was said at the 

 outset, do present, even externally, a certain degree of 

 similarity, the thigh, leg, and foot of the lower limb 

 evidently more or less repeating the upper arm, arm, and 

 hand of the upper Hmb. 



Mr. Herbert Spencer, in his First Principles of Biology, 



VOL. II. R 



u^ 



