MAMMALIAN SEXUAL CHARACTERS 147 



The so-called descent of the testicles occurs ex- 

 clusively in Mammals, in which with a few important 

 exceptions it is universal. This is a very remarkable 

 case of the change of position of an organ in the 

 course of development. The original position of the 

 testis on either side is quite similar to that of the 

 same organ in birds or reptiles. The genital ridge 

 runs along the inner edge of the mesonephros, with 

 which the testicular tubules become connected. 

 The testis, with the mesonephros, forming the epi- 

 didymis, closely attached to it, projects into the 

 coelom, and without losing its connexion with the 

 peritoneum changes its position gradually during 

 development, passing backwards and downwards 

 until it comes to lie over the wall of the abdomen just 

 in front of the pubic symphysis of the pelvic girdle. 

 There the abdominal wall on either side of the middle 

 line becomes thin and distended to form a pouch, 

 the scrotal sac, into which the testis passes, still 

 remaining attached to the peritoneum which lines 

 the pouch, while the distal end of the vas deferens 

 retains its original connexion with the urethra. The 

 movement of the testis can thus be accurately 

 described as a transposition or dislocation. 



Various causes have been suggested for the forma- 

 tion of the scrotum, but no one has ever been able to 

 suggest a use for it. It has always been quite im- 

 possible to bring it within the scope of the theory of 

 natural selection. The evolution of it can only be 

 explained either on the theory of mutation or some 

 Lamarckian hypothesis. The process of dislocation 

 of the testis does not conform to the conception of 

 mutation, nor agree with other cases of that pheno- 

 menon. A mutation is a change of structure affecting 



