196 THE FORMATION 
purpose, solio “seconds” are used, since they are both cheap and satisfactory. 
When an urgent demand for a finished print does arise, it is met by using 
“velox” paper, which can be expcsed in the dark room, and then developed 
and fixed exactly like a plate. Two standard papers for views are “solio” 
and “platina.” The former gives brown tones, and is used for contrast and 
brilliancy, hence it is especially good for printing from negatives that have 
too much detail and too little contrast. ‘Platina,’ on the contrary, yields 
soft gray tones, and softens contrasts. 
FORMATION AND SUCCESSION HERBARIA 
243. Concept and purpose. A formation herbarium is a collection of 
exsiccati, in which the species are arranged with respect to their position 
in the formation, instead of being grouped in genera and families. Its 
primary purpose is to furnish a record of the constitution and the structure 
of a formation or a series of formations. At the same time, it affords the 
basal material for developing the subject of comparative phytogeography. 
It is impossible for one ecologist to visit many remote regions, to say noth- 
ing of spending a period sufficient for obtaining even a fair knowledge of 
the vegetation. He can at the best acquire an acquaintance with but few 
regions at first hand. In consequence, a method that brings a vegetation to 
him, with its structure carefully wrought out by years of study, is of the 
highest value. Time, as well as distance, sets a narrow limit to the number 
of formations which one man can investigate critically in a lifetime. It 
is no longer possible for a botanist to explore vast regions, and to bring 
back results which have anything more than a very general value. This 
fact, far from restricting the comparative study of vegetation, will serve 
to make it more accurate and systematic. The exact results of numerous 
resident investigators, expressed in formation herbaria, with the proper 
series of quadrat maps and photographs, will be worked over by men who 
are themselves specially acquainted with a particular vegetation. Compari- 
sons will be founded upon a definite basis, and the relationship of various 
vegetations can then be expressed in precise rather than general terms. It 
is hardly too sweeping to assert that accurate work in the field of compara- 
tive phytogeography can be done only in this fashion. The value of forma- 
tion herbaria_in class work is evident. On account of the limitations of 
time and distance, classes can touch but few formations, and these at every 
time except the growing period. For these reasons, an accurate and com- 
plete formational record that can be consulted or studied at any time is 
almost indispensable to class study in the development and structure of 
formations. 
