THE VOICE OF THE DESERT ^O 



like its desert cousin, but its flower is much like theirs, 

 while that of the ocotillo is different. 



Of course it would be nice if we had a complete series 

 of dated fossils which would illustrate the emergence in 

 time of the relatively superficial characteristics of the 

 desert cactus. In the case of various animals, such series 

 do exist, notably in the case of the horse, whose evolution 

 was illustrated quite early in the study of evolution and 

 whose gradual loss of toes can be neatly demonstrated in 

 the successive cases of a museum. Unfortunately, however, 

 nothing of the sort is possible in the case of the cacti. 



In the first place the whole order of flowering plants 

 may have evolved rather late. In the second place deserts 

 are not very favorable to the preservation of fossils, which 

 occur most abundantly in what were once marshes or 

 shallow seas. As a matter of fact, it is sometimes said that 

 there are no fossil cacti and that therefore they must be 

 very recent indeed. But this evidence is purely negative 

 and a decade or two ago what may be a fossilized- prickly 

 pear was discovered in southern Utah embedded in de- 

 posits of the Eocene age. If it really is a prickly pear, then 

 a cactus very like a modern one must have developed 

 during an epoch which, according to the latest evidence, 

 must be placed at least forty million years ago. 



It is not even certain whether the drought-resisting de- 

 vices were developed by plants growing in a region which 

 gradually dried up, or whether they were developed as 

 the plants increased their range into more and more arid 

 regions. But if, by chance, the place where the one fossil 

 of a prickly pear was found is the place where it developed 

 its peculiarities, then it seems likely that the process was 

 one of adjustment to a changing climate because, so the 



