282 THE CASE AGAINST EVOLUTION 



nephros persists for a few months after hatching in the adult, 

 the definitive kidney of the adult being reinforced during that 

 interval by the still functional mesonephros. In anamniotic 

 vertebrates, however, no separation exists between the meso- 

 nephros and the metanephros, the two forming one continuous 

 structure, the opisthonephros, which acts as the functional 

 kidney of the adult. 



(3) The metanephric system: In the amniotic vertebrates 

 the mesonephros and metanephros are distinct, the former 

 being functional in embryos and in adult reptiles (for a few 

 months after hatching), while the metanephros becomes the 

 definitive kidney of the adult. The metanephros is a collec- 

 tion of nephridial tubules provided with a special urinary 

 duct called the ureter, which empties into the bladder (not 

 the cloaca). The Wolfiian or mesonephric duct is retained 

 as a sperm duct in the male (of amniotic vertebrates), but 

 becomes vestigial in the female. Only a certain number of 

 the nephridial tubules of the embryonic metanephros are taken 

 over to form part of the permanent or adult kidney (in mam- 

 mals, birds, and reptiles). 



If, then, as we have previously observed, we follow Kerr in 

 regarding the fish kidney, not as a simple mesonephros, but 

 as an opisthonephros {i.e. a combination of mesonephros and 

 metanephros), there is no warrant for interpreting the embry- 

 onic mesonephros of man and mammals generally as the fish- 

 kidney stage. But waiving this consideration, and assuming, 

 for the sake of argument, that the fish kidney is a perfect 

 homologue of the human mesonephros, the mere fact of the 

 adoption by the human embryo of a temporary solution of 

 its excretory problem similar to the permanent solution of 

 that problem adopted by the fish, would not, of itself, imply 

 the common ancestry of men and fishes. Such a coincidence 

 would be fully explicable as a case of convergent adaptation 

 occurring in the interest of embryonic economy. 



It is, indeed, a well-known fact that larval and embryonic 

 organisms are often obliged to defer temporarily the construe- 



