150 MAMMALIAN SEXUAL CHARACTERS 



It is evident that the violent shocks of the gallop- 

 ing or leaping movement do not occur in Birds, 

 Reptiles, or Amphibia. Ostriches run very fast and 

 do not fly, but their progression is a stride with each 

 foot alternately, not a gallop. The Anura among 

 the Amphibia are saltatory, but their leaps are 

 usually single, or repeated only a few times, not 

 sustained gallops. The exceptions among the 

 Mammalia still more tend to prove the close corre- 

 spondence between the ' impulsive ' mode of pro- 

 gression and the dislocation of the male gonads. 

 In the Monotremata there is no scrotum, the testes 

 are in a position similar to that which obtains in 

 Reptiles, and they are the only Mammals in which 

 these organs are anterior to the kidneys. In 

 locomotion they are sluggish, there is no running or 

 galloping among them. Ornithorhynchus is aquatic 

 in its habits, and Echidna is nocturnal and moves 

 very slowly. In Marsupials the scrotum is in front 

 of the penis, but really in the same position as in 

 other Mammals that is, in front of the ventral part 

 of the pelvic girdle. It is the penis which is different, 

 as the skin around the organ has not united in a 

 ventral suture below it, while the organ itself has not 

 grown forward adnate to the abdominal skin as in 

 most other Mammals. The scrotum is always 

 anterior to the origin of the penis, although in the 

 Eutheria apparently behind that organ. The larger 

 Marsupials like the kangaroos are eminently 

 saltatory, and the others are active in locomotion. 

 The aquatic Mammals Sirenia and Cetacea have no 

 scrotum, the testes being abdominal. It is un- 

 necessary to inquire whether this is the original 

 position, or whether they are descended from 



