(J3 ^^^ t^^y 9°^ *^<^^ way 



spread were closed in one way or another. The classic 

 example of a similar phenomenon in the animal world is 

 the nearly complete absence of native mammals in Aus- 

 tralia despite the fact that they are native to every other 

 continent. Obviously their ancestors, the marsupials, had 

 got to Australia across some bridge which had disappeared 

 before the marsupial stock had evolved true mammalian 

 form somewhere on one of the continents by then isolated 

 from Australia. 



No botanist would say today that every endemic family 

 or genus is bound to be younger than any cosmopolitan 

 one. It is not so simple as all that. For one thing the 

 existence of "discontinuous distributions" (i.e., genera en- 

 demic in several widely separated regions) is sufficient 

 to suggest that an endemic genus may sometimes be 

 merely the relict of a cosmopolitan one which has be- 

 come extinct except in a few places. But if a plant found 

 in one place never did exist anywhere else, then it prob- 

 ably originated fairly recently and somewhere not too far 

 from where it is now found. Something like that is prob- 

 ably (but only probably) the case with the cactus. 



In any event, most would probably agree that this much 

 may be said with a fair degree of assurance: the cactus 

 originated, either in Mexico or South America, as a normal 

 enough plant with slender stems and orthodox leaves like 

 one member of the family (Pereskia) still growing in the 

 West Indies and South America. That ancestral cactus 

 may have been accustomed then to a fairly moist en- 

 vironment and it is in such an environment that it still 

 flourishes. But from a remote period it has always been 

 distinguished by the anatomical peculiarities of the family 

 — notably by a certain floral structure and the presence 



