March 3, 1870] 
WATURE 
455 
small matters ; but I hope I have shown that, so far as 
we have any means at present of measuring geological 
time, they may be amply sufficient. Taking Sir William 
Thomson’s allowance of a hundred million years for the 
time during which the earth can have been fit for life, it 
yet allows Mr. Darwin, for the process of development 
from the primordial germ, three times as many years 
anterior to the Cambrian epoch as have elapsed since 
that date, an amount of time which, I believe, will fully 
satisfy him, by whatever scale we may measure it. 
The evidence of the rapidity of denudation would indeed 
tend to the still further shortening of the estimate here 
given ; and it is not impossible that a concurrence of 
geographical conditions might have brought down the 
glacial epoch into a period when the excentricity was no 
greater, or even less, than it is now. This, however, is 
hardly probable, and I am inclined to think that the con- 
siderations already alluded to will, to a considerable 
extent, explain how it is that so many signs of glacial 
action still remain, in spite of such denudation. The 
only argument I consider new in this paper, is that de- 
rived from the uniformity of climate during the last 
60,000 years, and the alternations of heat and cold for a 
long time previously, leading to a slower change of species 
since the glacial epoch than at any former period, 
thus allowing us to suppose change of form in the organic 
world to go on more rapidly than we had before thought 
possible. If this be a sound deduction, it will, I believe, 
more than anything else, enable us to bring the period 
required for the development of the whole organic world 
within that which modern natural philosophy assigns as 
the age of the habitable earth. 
Much of the force of my argument appears to depend 
upon the accuracy of Mr. Croll’s view, that, during a time 
of great excentricity, there will be in each hemisphere 
alternately a glacial epoch for about 10,500 years, and a 
perpetual spring or summer for about an equal period. 
But Sir Charles Lyell argues, with great force, for the oppo- 
site view, that the cold of one period would be continued 
through the other, and that during the whole continuance 
of a phase of high excentricity both hemispheres would 
be in a state of glaciation. Supposing this view to be the 
true one, it will not very materially affect my argument, 
for the diagram shows many comparatively rapid alterna- 
tions from a very high to a very low excentricity, which 
would also be from a glacial to a temperate climate and 
would certainly tend to comparative rapidity of specific 
change; while in each 10,500 years there would, no 
doubt, be some retreat and advance of the snow line, 
followed by a less amount of migration, competition, and 
variation: During the last 60,000 years, on the other 
hand, the change of excentricity has been hardly per- 
ceptible, and the change of organic forms may be sup- 
posed to have been far below the average. 
ALFRED R. WALLACE 
FRESHWATER CRUSTACEA OF NORWAY 
Histoire Naturelle des Crustacés d'eau douce de Norvege ; 
by George Ossian Sars. Part I. Malacostraca. With 
1o plates. (Christiania, 1867.) 
S OME few years ago, much interest was excited amongst 
naturalists by the announcement of the occurrence, 
in the great Swedish lakes Venner and Vetter, of certain 
Crustacea, heretofore known only as marine species in- 
habiting the Arctic and Baltic Seas. The author of this 
discovery was Professor Lovén ; and the explanation of it 
appeared to be, that the gradual elevation of the Scandi- 
navian peninsula had cut off these originally marine 
creatures from their natural habitat, and that they had 
been able to accommodate themselves successfully to 
altered conditions of life. 
In the volume now under notice, we have an elaborate 
—ve may say almost an exhaustive—contribution to the 
natural history of these and other fresh-water crustacea of 
Norway. The species here treated are JZyszs oculata, var. 
relicta; Ganunarus neglectus; Pallasta cancelloides, var. 
guadrispinosa, Gammaracanthus loricatus, var. lacustris ; 
Pontoporeia affinis; and Asellus aguaticus. One notices 
with surprise the absence of the most abundant fresh- 
water Amphipod of our own country, Gammarus pilex, 
and its replacement by the very closely allied G. neglectus. 
The well-shrimps (4Vzfhargus) seem also to be unnoticed 
as yet in Norway; neither do we find any mention of 
another group, inhabiting chiefly brackish water, but in 
some districts of England reaching into situations which, 
though affected more or less by tides, are yet of quite 
fresh-water character ; e.g. Palemion varians and Mysis 
vulgaris. 
The anatomy and physiology of all these animals, and 
their points of variation from the typical marine forms, are 
most carefully and elaborately worked out. The following 
interesting remarks occur respecting Pontoporeza affinis. 
The males of this species are numerous, having their 
antenne either imperfectly developed or presenting a very 
peculiar form, except in some individuals, where, towards 
the end of autumn, the antenne take on their fully developed 
form; in both cases, the animals being perfectly fertile. 
This phenomenon is analogous to some already observed 
in the Cumacea and Tanaide. The great likeness between 
this and the Greenland species, P. femorata, led the 
author at one time to consider the two as presenting only 
varietal differences, the Norwegian species exhibiting a 
permanent arrest of development such as he shows to be 
the case with the variety ve/écta of MJysts oculata. This 
supposition, however, he was compelled to dismiss; one 
important fact tending to a contrary opinion being that 
the secondary appendage of the superior antenna contains 
in P. affinds a larger number of joints than in P. femorata. 
We would commend the dredging of our deeper lakes to 
the attention of English naturalists. Nothing in that 
direction has, so far as we know, been done in this country ; 
and it is worthy of remark, that only in the very greatest 
depths of the Scandinavian lakes were the abnormal 
species found. That this field is not unlikely to prove a 
productive one near our own doors we fully believe, inas- 
much as we have ourselves found, and elsewhere pub- 
lished, some interesting instances of the occurrence of 
truly marine microscopic crustacea in fresh water in the 
west of Ireland. 
It should be added, that the plates illustrating M. Sars’ 
work are admirable specimens of the engraver’s art, and 
leave nothing to be desired as to copiousness and accuracy. 
The work is altogether well produced ; the expenses, which 
we fear are scarcely likely to be repaid, having been gene- 
rously borne by the publisher, M. le Réviseur d’Etat 
Johnsen. GS. B, 
