140 UNIVERSALITY OF DEGENERATIVE EVOLUTION 



phros as the organ for eliminating urine, but, at 

 the same time they make use of the metanephros 

 which is also functional. 



In Chamceles the mesonephros remains partially 

 active throughout life. Both birds and mammals 

 completely lose the mesonephros, and in the adult 

 stage the metanephros is the only active kidney. 



This is not the place in which to complete our 

 study of the recapitulation theory, and we shall 

 have to recur to it later on ; but it was impossible 

 to describe the numerous rudimentary organs of 

 this system without taking a comprehensive glance 

 at the individual and specific development of the 

 whole. This investigation, moreover, raises another 

 question. 



It has just been shown that the epididymis is 

 only a vestige of the mesonephros, but in this case 

 it cannot be said that there has been degeneration ; 

 what has happened is that an organ has been trans- 

 formed, and that one function has been replaced by 

 another. According to some authorities the supra- 

 renal capsule, an organ of unknown, but doubtless 

 essential function, is the result of the transformation 

 of the pronephros. If this theory be ultimately 

 established, it will furnish a second example of what 

 we have stated above. The thyroid gland may 

 and ought to be investigated from this standpoint. 

 The various component parts of this organ had 

 no original connection. In higher vertebrates only 

 the central part of the organ appears to be similar 



