138 ANIMAL LIFE IN DESERTS 



smell ; but Hart noted that they would eat Daemia, 

 a very milky Asclepiad. I have seen goats at Jericho 

 climbing up to browse on Zizyphus twigs though 

 the spring herbage was up to their bellies. 



Furthermore, thorniness is most highly developed 

 in the most arid deserts, exactly where large grazing 

 animals are rarest, and the plants of the American 

 deserts are on the whole more thorny than those of 

 the Great Palsearctic Desert, though wild grazing 

 animals are rare in the American deserts. 



The majority of the cacti are covered with long, 

 straight thorns, which mice use as if they were 

 rungs on a ladder ; the fruits (e.g. in Opuntia) carry 

 small tufts of minute barbed hairs, which are easily 

 detachable and very sharp ; these are thickly set 

 on the fruit, and it might be supposed that they would 

 form an efficient defensive mechanism, but this is 

 far from being the case. In the American deserts, 

 in which these plants are native, the squirrels and 

 mice eat the fruits in great quantity. In the Old 

 World, where Opuntia is an introduced plant, the 

 rodents have learnt to climb the plants and steal 

 the seeds. At Jaffa two species of the genus are 

 cultivated : the fruit of one is eaten by man ; that 

 of the other is opened by Gerbils (Gerbillus allenbyi), 

 which gorge themselves and leave very few seeds 

 to germinate. With the spiny fruit of Opuntia we 

 may contrast the native and apparently defenceless 

 gourd of Citrullus colocynthus, the Colocynth, which 

 is not eaten by rodents or ruminants or jackals,! 

 apparently because it is bitter and violently] 

 purgative, and the Water Melon (C vulgaris), which 



