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THE POPULAR SCIENCE MONTHLY. 



probably more striking force than obtains in any other single 

 instance throughout the range of flowering plants. 



Only within the memory of men still in the prime of life has 

 the full significance of this Southwestern flora dawned upon the 

 world of science. Far back in the history of early explorations 

 travelers and naturalists had recognized the odd character of these 

 north-Mexican plant forms, but to realize their inward meaning 

 required the elaborate monographing of Engelmann and the broad 

 generalizing of Asa Gray. For, strange as it may seem, one in- 

 vestigator after another, enthusiastic over the rich flora spread in 

 such profusion from our Atlantic seaboard westward beyond the 

 Rockies, nevertheless shunned, because of the many difficulties 

 presented, this threefold group of Southwestern vegetation. Yet 

 this, above all else, was a flora peculiarly American — originating, 

 so far as we have yet been able to discover, on American soil, and 

 belonging to America alone. So here there was a prospect of 

 opening up to science a new aspect of plant life, and in due season 

 the men came with the opportunities and inclination to accomplish 

 the task. Foremost of all, and more than all the rest, stood forth 

 the St. Louis physician, Dr. George Engelmann, a skilled man of 

 medicine, with botanical inspiration. In him there seemed to be 

 an especially keen appreciation of the opportunity offered for 

 vastly aiding the cause of botanical science by the systematic 

 study of little-known groups of plants ; and through labors of this 

 nature, in addition to his note as a physician, he placed his name 

 among the greatest of monographers in the annals of botany. 

 And to him belongs the credit of turning the full light of science 

 upon the cacti, the agaves, and the yuccas, while through his in- 

 vestigations of these types the attention of our great American 

 systematist, Asa Gray, was first directly turned to the vegetation 

 of the Southwestern highlands. One of the absorbing problems 

 of Gray's life-work was what he once fitly termed "botanical 

 archaeology " — the study of the geographical sources of our wealth 

 of flora, and of the paths by which it had passed from one region 

 to another. Years of experience had enabled him to propound 

 the masterly theory of the great wave of ancient plant life sweep- 

 ing down from the north and giving to the Old World and the New 

 floras that have so many types in common. But later, largely in 

 the light of Engelmann's revelations, Gray was brought to fully 

 realize that a second great source of the peculiar elements in our 

 flora lay in the Southwest, down on to the Mexican plateau, and 

 beyond the reach of the influence of the Glacial age. Here was 

 the possible source of a vegetation strictly American, and to it 

 might be traced many now widely scattered tribes, but particu- 

 larly and most obviously the three unique types we are especially 

 considering. These have come down to us, in the land of the 



