part of all the known yuccas, altogether forming one of the most complete collections in the world of these Southwestern types; and he who carefully examines it will be ready to acknowledge it one of the most fascinating of all plant collections. Surely it could have no more fitting home than there in the city of Engelmann; and we can not but cherish the hope that no pains will be spared to make the collection even far more complete than it is, and thus give the American botanist a still greater laboratory in which to investigate so great a factor in American plant life. A suggestion of this aspect of the Missouri Garden may be found in the illustrations accompanying the present paper, most of which are from photographs taken there in July, 1892. The magnificent collection of cacti and the several flowering plants of agave in the World's Fair are of the highest interest in this connection. Can not these form a nucleus for a great permanent cactus garden?
From the general discussion we may appropriately pass to a more detailed sketch of each of the three groups before us, and in taking them up it will be found most convenient to place them in the order of their evolutionary rank: the cacti first, as representing the higher class of flowering plants; then the agaves; and lastly the yuccas, as somewhat lower in station than the second. This will have the merit, in addition to its logical virtue, of disposing of the weightiest group first, and of leaving till the last an amazing little entomological-botanical romance which gathers round the yucca. And the stately agaves will be not inharmoniously sandwiched between their two odd brethren. But let this suffice for a prospectus; the story will tell itself more satisfactorily.
Viewing the three members of our group together, the query presents itself: Is there not some vital significance in the relative extent and diversity of development in these three joint monarchs of the desert? Two of them, those we shall consider later, reach only the magnitude of genera, each constituting a moderate-sized and not remarkably diversified genus; while on the other hand the cacti together form an immense family, the natural order Cactceæ, aggregating over a thousand species, gathered into a number of genera. It is but a grand example of evolutionary principles, "natural selection and the survival of the fittest," for the facts must be interpreted in the light of Darwin's immortal phrase. The yucca pushes its sturdy rootstock through the sand and drinks up each available drop of water; the agave's succulent leaves store up a wealth of nutritious sap; but the cactus seems to be pre-eminently an invulnerable storehouse of life-giving moisture, and the veritable offspring of the arid, rocky sand-wastes, while the others seem only adopted children. Mark the peculiar characters of the typical cactus: The
