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 investigator 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 investigations 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 archæology"—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 sweeping 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 particularly and most obviously the three unique types we are especially considering. These have come down to us, in the land of the
