ANIMALS— PHYSICAL ENVIRONMENT 73 



good winter rains, he travelled along the same road, 

 at the same season. He saw dozens of white 

 butterflies between Biskra and Borj Saada, and 

 hundreds of Melitaea and Euchloe between that place 

 and Tuggurt. 



Spencer's observations upon the small mammals 

 and other creatures of Central Australia, and the 

 effect upon them of " good " and " bad " years, are 

 extremely interesting. After the wet season the 

 reproduction of the small rodents and marsupials 

 is at its height ; the succeeding drought kills a very 

 large number of them, especially the immature 

 individuals, and the few survivors attain maturity 

 without attaining the dimensions of individuals 

 reared under more favourable conditions ; for 

 instance, the adult males of the marsupial Phascogale 

 cristicauda may vary in length from 136 to 220 

 millimetres, and females carrying young from 125 to 

 170 millimetres. A succession of dry years not only 

 greatly reduces the numbers of small mammals, but 

 also reduces their powers of reproduction. Thus 

 the small, mouse-like marsupial Sminthopsis crassi- 

 caudata, which has typically ten nipples, produces 

 ten young at a birth in seasons of plenty, and not 

 more than four or five in seasons of scarcity. " If 

 the drought be abnormally prolonged then even the 

 hardiest animals will suffer, and the fauna wiU be 

 so reduced that it may take some time before 

 increased fertility on the part of the survivors and 

 the influx of immigrants from the broad belt of land 

 enclosing the central region will make good the 

 deficiency " (Spencer). 



