48 ELEMENTS OF BIOLOGY. 



longitudinal succession of bony segments which are formed 

 upon a common structural plan, and exhibit essentially the 

 same parts, though modified in different regions of the 

 spine. Much more conspicuous, however, than the serial 

 homology of the segments of the spine, is the homology 

 presented by the fore and hind limbs of the Vertebrata (figs. 

 1 6, 17). These, in all instances, can be shown to be modi- 

 fications of a common plan that is to say, they consist of 

 parts which are fundamentally similar to one another, 

 though very often the limbs may discharge different func- 

 tions, and may thus come to differ considerably in structure. 

 Lateral homology consists in the structural identity of the 

 parts on the two sides of the body in any given animal. 

 When this identity is complete, the animal becomes " bi- 

 laterally symmetrical ; " or, in other words, exhibits similar 

 and symmetrical parts on the two sides of the body. Some 

 animals, however, never exhibit any lateral homology or 

 bilateral symmetry at any period of their lives ; and others 

 only exhibit it when young, and lose it more or less com- 

 pletely when adult. It has been endeavoured to show that 

 lateral homology is the result of the similar way in which 

 conditions affect the right and left sides of the body respec- 

 tively (Herbert Spencer) ; but this does not appear to be in 

 any way an adequate explanation. In the first place, there 

 are many animals which exhibit bilateral symmetry in their 

 superficial structures and appendages, but which show no 

 such symmetry in the immediately contiguous internal 

 organs ; though it can hardly be pretended that the effect 

 of similar conditions extends, say, an inch below the sur- 

 face, but stops short at that point. In the second place, 

 there are many animals which belong to types in which 

 bilateral symmetry is the rule, but which, nevertheless, 

 normally and regularly exhibit a want of symmetry, either 

 in their appendages or in their internal organs. Thus it 

 cannot be pretended that the conditions which affect one 

 side of a Lobster are different to those which act on the 



