In the fall of 1890 Dr. George Vasey, then Botanist of the
Department of Agriculture, arranged with me to prepare a revision
of North American Cactaceae. Owing to the peculiar difficulty of
preserving material the family was poorly represented, even in
our leading herbaria. To secure a large amount of additional
material in the way of specimens and field notes the Department
authorized me to visit the region of the Mexican boundary during
the summer of 1891. Preliminary to this exploration it was
necessary to examine the Engelmann collection of Cactaceae, in
the possession of the Missouri Botanical Garden. This
collection, supplemented by the continual additions made at the
garden, is by far the largest collection of skeletons and living
specimens in this country, and also contains the large majority
of our types.