14 | 
On the other hand, the variety of uses which must now be made of pre- 
served material in the course of our Station studies necessitates the frequent 
employment of the nicest methods of the histological laboratory, and a com- 
plete acquaintance, at least, with laboratory methods in general. 
Definite and precise comparisons of different aquatic localities with respect 
to their biology have first been made possible in some considerable measure 
by the comparatively recent introduction of more or less exact quantitative 
methods for the collection and determination of the biological contents of the 
water. These, commonly known as plankton methods, enable us also to 
study the biological history of any locality to which they are fully appleable, 
by making it possible to bring into close comparison the organic contents of — 
the water from day to day, from season to season, and from year to year. 
Unfortunately, these methods are not as yet capable of appheation to all 
aquatic forms in all situations, but have been used successfully only for the 
smaller plant and animal forms of the clear open water. 
By using always identical apparatus in a perfectly uniform manner for the 
accumulation of microscopic and semi-microscopie objects in such waters 
and preserving the product by identical methods, it is possible to make and 
keep collections which may serve aS a means to a precise comparison of the — 
mass of organic life in the waters studied, or of the number of individuals _ 
representing any selected species. 
In our own Station work these plankton operations have been earried 
forward from the beginning at all the substations where open water could be 
found in condition to permit the hauling of our plankton net, or, later, the { 
use of the plankton pump. A large number of quantitative determinations § 
of collections so made have been worked out in a way to give the actual Fi 
quantity of organic life in a cubic meter of water for each situation studied 
by this method, and enumerations of various forms have likewise been made 
under the microscope by methods such as to give us reliable data for a com- 
parison of the various waters with reference to the number of such forms in _ 
a cubic meter of water. Approximately quantitative collections have like- ‘ 
wise been made, wherever possible, in situations which do not permit the — 
use of this plankton apparatus, but with results far less reliable, of course, 
because based mainly on personal estimates, and made by the use of less 
precise and exhaustive methods. 
It seems not impossible that quantitative and numerical plankton work will 
be found to have a certain value as a ground of inference concerning the 
biological contents of water which cannot be searched by the plankton appa- 
ratus. In other words, definite, if general, relationships may be found to 
exist between the amount and composition of the plankton in the free and 
open water of a given lake or stream at any given time and the mass and 
variety of living forms contained in the marginal shallows or imbedded in 
the mud of the bottom. 
Considerable modifications have been made at the Station since it opened, 
in the details of the quantitative method, often forced upon us by the pecu- 
harities of the.location and the special conditions under whieh our work was 
