JANUARY 19 



ceivable, is it not, that certain verminous insects found 

 the marsupium a convenient harbour, whence the 

 mammal attacked could not eject them by scratching. 

 For instance, West Indian and West African travellers 

 and residents are painfully familiar with the disgusting 

 'jigger' (Sarcopsylla penetrans), a tropical flea which 

 swarms in the ground and foliage of swampy districts. 

 The females burrow into the feet and toes of human 

 beings, and having excavated a nest in the flesh, swell 

 to the size of peas, and produce a large progeny of 

 maggots. Unless the nest and its contents are extracted 

 before the brood is hatched, serious ulceration is set 

 up, and an ugly wound is the result. It does not require 

 much imagination to conceive the effect upon the mar- 

 supium if it were to become the favourite nesting-place 

 of jiggers or insects of similar habits. Either marsupial 

 animals would die out and make way for those of a 

 type less vulnerable by vermin, or their structures would 

 become modified to suit the peculiar environment, and 

 the placental type would be evolved. 



The marsupial mole itself offers a striking example of 

 the effect of environment upon structure. In all other 

 known marsupial mammals the orifice of the characteristic 

 pouch is always directed forward, toward the head of the 

 creature ; but in Notoryctes it is turned the other way, 

 toward the tail. Why? The reason could scarcely be 

 plainer. The animal finds its living in fine sand, hunting 

 its prey rapidly through that irritating medium. If the 

 pouch opened towards the front, it would soon be filled 

 with sand as the mole burrowed its way underground. 

 It was a simple expedient, therefore, to turn it the other 

 way, which enables the young to sit at ease ' with their 



