RELATIONS OF ANIMALS AND PLANTS 125 



I that a very large number of species of bees, fos- 

 sorial wasps, ruby wasps, and true wasps congre- 

 gated at whatever plant was in bloom. The 



pHymenoptera could not afford to indulge their 

 preferences ; every species which was on the wing 

 might be taken now in a field of beans in flower, 

 a few weeks later at Zizyphus blossom, or Acacia 

 (Prosopis), or Lycium ; many of the species remained 

 on the wing during long periods, and a particular 

 species might frequent half a dozen different flowers, 

 each one exclusively for a short season. Such a 

 thing does not, I think, happen in lands where 

 flowers are commoner and the flowering season is 

 more prolonged. 



The inroads upon the annual vegetation of the 

 larger grass-eating mammals, for instance, the 

 gazelles and hares in the Old World, are relatively 

 unimportant because the numbers of these animals 

 are limited by the scarcity of fodder during the 

 dry season. Spencer showed that adults of the 

 marsupial Phascogale cristicauda in Central Australia 

 were 40 per cent, to 60 per cent larger at the close 

 of a good season following rain and plenty of vege- 

 tation, than they were in a season in which rain 

 had been deficient. He beheves that this explains 

 the great variation in size which may be observed 

 among adults of other species of small marsupials. 

 It is only such small beings as the insects, with their 

 power of lying dormant in unfavourable seasons 

 and of rapid multiplication, which can take advan- 

 tage of the annual flora of the desert. 



The plants of the second biological group, those 



