62 COMPARATIVE ANATOMY. 



§ 52. 



An efficient cause for metamerism may be sought, as has beeu 

 above indicated, in the phenomena of growth. We can imagine a 

 repetition of local outgrowths, resulting in practical advantage to the 

 organism, taking place in particular systems of organs simultaneously 

 with the elongation of the body. In this way the external meta- 

 merism may be brought iuto connection with the movement of the 

 body, which was perhaps the earliest cause of this phenomenon. 

 Many facts point to its being so. In any case there are numerous 

 examples of the gradual elaboration of metamerism without all 

 systems of organs being at once affected by it. Metamerism has, 

 however, a less doubtful origin in its connection with gemmation, 

 which is itself a kiud of growth. It seems, indeed, in many cases, as 

 if gemmation led to metamerism, in such a way that the metameres 

 represent buds, which remain connected with the organism, and only 

 in some cases attain to a higher stage of individual existence. 

 Numerous instances of incomplete metamerism prevent us, however, 

 from attributing a general significance to this process, and it cannot 

 in any sense be regarded as the sole cause of metamerism. 



Metamerism leads to perfection of the organism. By it 

 the organism is enabled to get a larger number of organs, although, 

 indeed, they are at first mere repetitions of one and the same 

 arrangement. As the separate segments become more independent 

 their action becomes more free, till at last the differentiation of a 

 larger number of separate organs gives a larger scope for action. 

 Differentiation, then, gains ground in every part, and alters the 

 organs of the separate metameres in different ways, according as 

 their functions become more various. By the development and 

 reduction of metameric organs the metameres get to differ in value, 

 and become differentiated themselves ; this differentiation is expressed 

 externally by the difference in their size and form. This leads to 

 the disappearance of the primitive similarity of the metameres. The 

 amount, too, of their independence may be lessened, and a number 

 of primitively separate metameres may gradually fuse into larger 

 divisions. This gives rise to complexes of metameres, in which the 

 fact of their being composed of separate units of the body is only 

 suggested, and that often faintly ; sometimes a large, sometimes a 

 small number of segments undergo concrescence. This, too, is on 

 the whole a cause of differentiation of the organism, as the body 

 consists in consequence of some independent and of some fused 

 metameres. Finally, metamerism may altogether disappear, and 

 the presence of separate organs alone indicate, and that often 

 obscurely, the phenomenon which obtained in the primitive state. 

 Every stage in metamerism is therefore a source of variation in the 

 external and internal organisation of the organism. 



