10 ANIMAL LIFE IN DESERTS 



in the deserts of India and in some of those of North 

 America are much less rigorous than they are in 

 most parts of the Great Palsearctic Desert : firstly, 

 because the season of great heat is not also the 

 season of great drought ; secondly, because the 

 periods of drought are shorter as there are two dry 

 and two wet seasons in the year ; and, thirdly, 

 because in the majority of stations a little rain falls 

 in nearly every month of an average year. If the 

 rainfalls which are shown graphically in Figs. 1-3 

 are studied it will be seen that either one or two 

 dry periods per year is characteristic of all the 

 places. In certain places these dry periods are 

 much more extended, for the precipitation is quite 

 irregular in its occurrence, and an intense desert 

 results. Hayward found mimosas dead at Kidal in 

 the South Sahara after five years without rain ; 

 one must suppose that, as trees had been able to 

 grow, the normal conditions were not utterly 

 unfavourable to plant life, but an unusually long 

 dry spell killed, no doubt, not only the mimosas but 

 other plants as well, and with each plant species a 

 host of insects, spiders, and other animals dependent 

 on it. A year after Ha3rward made his journey 

 across the Sahara Hartert was in Tidikelt, in the 

 Central Western Sahara (1912). He found that 

 rain had been very deficient for twenty years and 

 as a result the Chenopodiaceous plant Traganum, 

 which Rohlfs found growing in thickets in 1864, 

 had died completely, even to the very roots. 



Mrs. Forbes has recorded that when she was at 

 Kufara (Libya) in 1921 no rain had fallen for seven 



