THE VOICE OF THE DESERT 



62 



istic representative. They independently developed almost 

 precisely the devices which seem typical of the cacti, 

 though no North American species of Euphorbia ever 

 did. Some of the African sorts have thick, columnar, leaf- 

 less stems, waxy coatings and defensive spines, all of 

 which make them look so cactus-like that even an amateur 

 who had got beyond the stage, of calling an ocotillo a 

 cactus might be forgiven if he took it for granted that 

 at least these African Euphorbias really were cacti. But 

 they are not. Their flowers are totally different from cactus 

 flowers, and they have the thick milky juice which is not 

 characteristic of the cacti but is characteristic of the Eu- 

 phorbia family as a whole and is, indeed, responsible for 

 the utility of the economically most valuable members of 

 the family — namely the rubber plants. 



If then, the fact that the true cacti are all American does 

 not mean that cacti cannot live anywhere else and does 

 not mean that only in America did plants develop drought- 

 resisting devices similar to the cactus, then what on earth 

 does it mean? 



No one can answer that question with absolute finality, 

 but what it probably does mean is that the cactus family 

 not only began in America, but began quite recently — 

 as such things go; much more recently than, for example, 

 the milkweeds and the Euphorbias. And the reason for 

 this deduction is not hard to understand. 



Obviously the cosmopolitan families migrated somehow 

 from one continent to another. As early as the days of 

 Darwin and Wallace it was assumed that such families 

 were older than the "endemic" which, in some cases at 

 least, were endemic simply because they did not arise 

 until after the routes along which the other groups had 



