57 1^°^ *^^y 9°^ ^^cjt way 



slightly arid and some of those which do so, do not look 

 at all like what we think of when we hear the word 

 "cactus." They have quite ordinary leaves and stems. In 

 fact they are not very unusual in appearance. And the 

 assumption is that it was some such quite orthodox mem- 

 ber of an orthodox family which learned to live with less 

 and less water as it turned itself into a desert species. 



Now, at some of these statements the layman may well 

 bristle. "Why on earth," he may ask, "do you tell me that 

 the ocotillo — which has thorns, green stems, and only 

 small temporary leaves — is not a cactus, while some quite 

 ordinaiy-looking plant which lives in a reasonably damp 

 environment is? Why wouldn't it be more sensible to say 

 that the ocotillo and the cactus are alike, that they de- 

 scended from some common ancestor which was, and 

 perhaps always had been, desert dwelling and cactus- 

 like?" 



The answer to that protest is very much like the answer 

 to the question, "Why call the road runner a cuckoo?" 

 You call him that because of certain fundamental ana- 

 tomical characteristics which, as we know from fossil and 

 other evidence, change much more slowly than the super- 

 ficial peculiarities which make him seem at first glance 

 uncuckoo-like. 



All botanists, whether they be students of the living 

 species or of fossils, agree that the anatomy of the flower 

 is one of the more stable characteristics of a plant group. 

 Classification is based upon all the characteristics of a 

 plant, and the flower structure and the obscure anatomy 

 of all desert cacti are similar, while they are different from 

 those of the ocotillo and other superficially similar plants. 

 The tropical cactus of a moist environment may not look 



