59 ^"^^ ^'^^y 9°^ ^^'^^ way 



geologists say, the Great Basin section of Utah became 

 drier and drier as the Eocene was succeeded by the Mio- 

 cene, and by that epoch it may have been as dry as Death 

 Valley is today. At least the best guess, based upon what 

 evidence is available, seems to be that the cactus from 

 Utah first learned somewhere how to survive dry seasons 

 of the year between relatively wet ones and that then, 

 having developed methods of saving water for a part of 

 the year, it was finally able to survive almost continuous 

 drought. And it might have done this right in Utah, since 

 it seems pretty certain that this very region where the 

 Eocene cactus was growing was a region which had be- 

 come arid all the year around by the time it entered the 

 next geological epoch. 



By way of compensation for the paucity of fossil evi- 

 dence bearing on the evolution of the cactus, there is 

 something we do have: a rather remarkably complete 

 series of living forms which illustrate very neatly many 

 of the steps by which the not-at-all-cactus-like ancestor 

 probably took on, one by one, the distinguishing char- 

 acteristics of the most peculiar and specialized kinds. The 

 eohippus or "dawn horse" is extinct. To illustrate his evolu- 

 tion we have only the fossils. In the case of the cactus 

 the situation is exactly reversed. A plant probably similar 

 to the "dawn cactus" is still growing in South America. 



Perhaps just because cacti are so odd, so unlike what 

 we expect a plant to be, they have been made the subject 

 of exhaustive study. In the great four-volume work by 

 Britton and Rose the living sorts are magnificently illus- 

 trated. In Berger s Die Entwicklungslinien der Kakteen 

 the probable lines of development are exhaustively studied. 

 Instead of the museum case showing the skeletons of 



