JUNE 21, 1901.] 
The final part of the work deals with what 
is denominated ‘ plant formations,’ and a plant 
formation is defined as ‘a piece of the floral 
covering, the extent of which is determined by 
a characteristic correlation or association of 
vegetable organisms, 7. ¢., it is a stretch of land 
the limits of which are biological and not 
physiographical.’ They may or may not, 
therefore, be coextensive with the regional dis- 
tribution of the plants which compose them. 
With respect to their origin they may be 
either primitive or recent. Primitive origin is 
necessarily more or less conjectural. It in- 
volves the study of the extinct flora of the 
region and the geologic changes which finally 
led up to the establishment of existing condi- 
tions. Recent formations may arise either 
from nascence or by modification. If by nas- 
cence they must originate upon areas previously — 
destitute of any floral covering, while in the 
second case they are formed by the elaboration 
or modification of existing formations, often by 
the intrusion of foreign elements. Abandoned 
cultivated patches may represent the first, 
timber claims the second. Formations often 
disappear through the agency of fires, floods, 
mankind, etc., in which cases new formations 
may arise by nascence. 
As an example of the latter is quoted the 
establishment of Botrydium granulatum or Vauch- 
eria sessilis, with the cup fungi Humaria and 
Scutellinia, on muddy flats, formerly occupied 
by water plants, but subsequently exposed by 
the drying up of ponds or streams. A carpet- 
like layer may then supervene, composed of 
Riccia glauca, Funaria hygrometrica, ete., which 
in turn may be replaced by low-growing flower- 
ing plants, such as Portulacca oleracea, Lepidium 
intermedium, etc., and this in turn may yield to 
taller growing Chenopodiums, Amaranths, ete. 
In the origination of formations by modifica- 
tion two sets of factors may be distinguished— 
natural and artificial. The first are either bio- 
logical or physical, the second are due to the 
influence of man or other animals. An un- 
usually wet season in the sandhill region or a 
dry one in the lowland will often modify the 
floral covering in a striking way, and modifica- 
tion through the agency of man is too obvious 
to require more than passing mention. 
SCIENCE. 
983 
The various plant formations recognized are 
finally considered in detail and discussed under 
headings and sub-headings, physiographical and 
biological. For example, under the forest for- 
mations may be found the river-bluff forma- 
tion, including (1) the red oak-hickory forma- 
tion, (2) the bur oak-elm-walnut formation, etc. ; 
under prairie formations, (1) prairie grass, (2) 
buffalo grass; under foothill formations, (1) 
under-shrub, (2) mat and rosette, (8) grass, 
etc., and many others. 
The work as a whole is exceedingly valuable 
for the wealth of facts recorded, irrespective of 
any conclusions which may be deduced from 
them, and no one can fail to appreciate the im- 
mense amount of conscientious labor which it 
shows. 
ARTHUR HOLLICK. 
Synopsis of the Naiades, or Pearly Fresh-water 
Mussels. By C. T. Stimpson. Proc. U. S§. 
National Museum, XXII. 1900. Pp. 501- 
1044. 
Some people think that the preparation of 
zoological catalogues and synopses is a low grade 
of work, which should be turned over to those 
who are not capable of doing anything better. 
Certainly one occasionally meets with examples 
of zoological bibliography bad enough to have 
been compiled by the most incompetent, butitis 
to be remarked that the authors of these works 
are often really skilled in anatomy or some 
other branch of the science. The fact is that 
the preparation of such a work as we have be- 
fore us, with its orderly arrangement of innu- 
merable references, requires not merely a high 
grade of intelligence, but a special kind of 
ability, which is none too common. We may 
therefore begin by thanking Mr. Simpson for a 
work which no other living man was equally 
competent to produce, and which will be in- 
valuable to all students of the naiades. But to 
regard the work as merely a piece of good bibli- 
ography, would be extremely erroneous. Mr. 
Simpson has carefully studied a considerable 
majority of the species, and the arrangement 
of them is original with him. He has, by an 
examination of the soft anatomy, been able to 
show that the genus Unio of authors isin reality 
a heterogeneous mass of distinct genera, which 
