CHAP. VIII.] LIKENESSES IN ANIMALS AND PLANTS. 257 



They 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 further 

 analysis, with respect to parts said to be homologous. 

 It came to be recognised that there are likenesses not TueT 

 between different animals and different parts of the &quot; 

 same animal, which a theory of common descent cannot ex 

 plain ; and &quot;similarity in relative position&quot; 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 &quot; descent,&quot; and in others not so explicable. 



A very obvious example of likeness not explicable by 

 &quot; descent &quot; is the familiar one between our right hand and our 

 left. This likeness is part of that general correspondence 

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

 and which is spoken of as &quot;bilateral symmetry,&quot; or lateral 

 homology. 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 &quot; vertical symmetry,&quot; or vertical 

 homology. Another kind of &quot;likeness,&quot; or homology, is 

 termed &quot; serial.&quot; It is chiefly in our limbs 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 (vertebra) which make up the vertebral 

 column or backbone. Our limbs, however, 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 limb. 



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

 attempts to explain these and all facts of structure, Mr Spencer . 9 

 not due to inheritance, by the action upon each ex P lanatlons - 

 organism of its environment. Thus he explains the very 

 general absence of symmetry between the dorsal and ventral 

 (upper and lower) surfaces of most animals by the different 



