414 
NATURE 
[Marcu 4, 1897 
Inorganic Chemical Preparations. By ¥. H. Thorp. 
Pp. 238. (Boston, Mass.: Ginn and Co., 1896.) 
THIS work is divided into two parts, the first being an 
ntroductory chapter on general chemical operations 
such as solution, precipitation and filtration, and the 
second containing detailed directions for the preparation 
of 100 inorganic salts. The instructions in the first 
paper are very minute, and are apparently intended for 
elementary students. In the second part the arrange- 
ment followed is alphabetical, and it is stated in the 
preface that no attempt has been made to observe any 
particular grouping or sequence in the preparations. 
Thus the first preparation described is that of an- 
hydrous aluminium chloride, and this is followed 
by aluminium hydrate and sulphate, preparations of 
quite another order of difficulty. This lack of arrange- 
ment and want of gradation seriously detract from 
the value of the book for teaching purposes. Little 
or no stress is laid upon the purity of the product, 
although the removal of one impurity is occasionally 
given, such as copper in the preparation of lead acetate. 
The preparation of pure iodine or silver, or even of pure 
water, according to the methods of Stas, would possibly 
be of higher educational value, and certainly be more 
interesting to the student than the formation, say, of 
‘barium and lead chromates by precipitation. <A few of 
the equations given for the reactions require some revision. 
Thus for aluminium chloride we find 
Al + Cl, = AICl,, 
‘but for phosphorus trichloride 
P + 3Cl = PCI. 
As a collection of recipes, the work will be handy for 
reference in the laboratory. The inclusion of the methods 
of preparing some common reducing agents, such as 
cuprous chloride, chromous chloride, and sodium hypo- 
sulphite, would have added to the value of the book. 
The Practical Photographer. Edited by Matthew Surface. 
Vol. vii. Pp. 332. (Bradford and London: Percy 
Lund, Humphries, and Co., Ltd., 1896.) 
THIS very attractive volume should be seen by all who 
are interested in photography. The illustrations in it 
are striking examples of what can be done in the way 
of reproducing illustrations by photographic processes. 
Of especial interest is the series of short articles on 
photography and photographers in Japan. The aim of 
the editor seems to be to show the best that photo- 
graphy is capable of, whether in art or science or com- 
mercial application, and he may be congratulated upon the 
successful way in which he carries out this programme. 
Life Assurance Explained. By William Schooling, 
F.R.A.S. Pp. xvi+ 185. (London: Cassell and 
Co., Ltd., 1897.) 
‘THE principles upon which life assurance is based are 
stated very clearly in this book, and without reference to 
the merits or otherwise of individual companies. The 
book presents some instructive points for elementary 
students of vital statistics and actuarial methods. It is 
also a practical and a trustworthy guide, which should be 
consulted by every one who contemplates taking out an 
insurance policy of any kind. 
Wasted Records of Disease. By Charles E. Paget. 
vill + 92. (London: Edward Arnold, 1897.) 
IN the three chapters of this book Mr. Paget reviews the 
attempts made to secure or establish permanent systems 
of disease registration ; legislative recognition of the 
need for such registration, and its shortcomings; and 
steps advisable to secure a permanent and useful system 
of national disease registration. His plea for the estab- 
lishment of a national system of notification and regis- 
tration of disease, will have the support of most practi- 
tioners and officers of public health. 
NO. 1427, VOL. 55 | 
Pp. 
LETTERS TO THE EDITOR. 
(The Editor does not hold himself responsible for opinions ex- 
pressed by his correspondents. Nether can he undertake 
to return, or to correspond with the writers of, rejected 
manuscripts tntended for this or any other part of NATURE. 
No notice ts taken of anonymous communications. | 
Specific Characters. 
Dr. A. R. WALLACE, in his extremely interesting paper on 
““The Problem of Utility,” lately published in Zzun. Soc. 
Fourn.—Zool., vol. xxv., arrives at the conclusion (p. 486) that 
“every species (of the higher animals at all events) will usually 
possess at least three peculiarities: in the first place, it must 
exhibit some difference of structure or function adapting it to 
new conditions ; secondly, some distinction of colour, form, or 
peculiar ornament serving as distinctive recognition-marks ; and, 
thirdly, the physiological peculiarity of some amount of infertility 
when crossed with allied species. The first two constitute its 
“specific characters.’ ” 
Now it appears that the first of these differences is the 
fundamental one, and we ought not to find species living under 
exactly the same conditions and in precisely the same manner, 
separated only by infertility or ‘‘ recognition-marks.” Yet any 
one examining the current literature of entomology, would sup- 
pose that in numerous instances there were no differences what- 
ever between allied species than those either of the class 
of *‘ recognition-marks,” or in the structure of the genital organs. 
That this would be an extremely erroneous supposition I am 
convinced both by experience and on theoretical grounds, and I 
would ask entomologists to produce even a single valid instance 
in support of it. The fact is, that the specific characters of the 
first class are overlooked by those who describe insects, until 
the describers come to imagine they have no existence. Nor is 
this surprising, since they are largely such as can only be 
elucidated by observations on the living insects, and no amount 
of cabinet-study will detect some of them. 
It follows from the above considerations that species may 
occur which are perfectly distinct, but nevertheless offer no 
palpable differences in dead specimens. I know several instances 
of this sort, they are what I have termed Ahyszologecal specees. 
As Dr. Wallace states, recognition-marks are practically universal 
among the higher animals, but there occur groups in which they 
could not be of much, if any, use; and here it is that the 
separation of the species becomes so intricate. It is fortunate 
that many groups in which recognition-marks are reduced to 
a minimum, the organisms are minute and often trans- 
parent, so that their whole structure can be seen under the 
microscope. 
In the case of insects, physiological species appear among the 
degraded forms, such as the Coccidee and the bird-lice. Thus 
the coccid Asprdiotus aurantz@ is a great pest of orange-trees in 
California, the Eastern Mediterranean region, &c., but in 
Jamaica occurs a form of it, not distinguishable structurally from 
the type, which never attacks the orange. Lately Prof. Kellogg, 
ina paper on bird-lice, stated that a certain so-called species 
had a great number of hosts, and probably consisted of several 
species, confined to particular genera or species of birds; but, 
nevertheless, all attempts to separate them on structural grounds 
had proved unsatisfactory and inconclusive. Both the coccids 
and the bird-lice are creatures in which recognition-marks could 
not be of much service. The males of many Coccidz, which 
are never seen by the females, are remarkably uniform in 
appearance, considering the structural diversities of many . 
of their mates, the latter having contrivances for protection 
against parasites, against too rapid evaporation or too great 
heat, for the protection of the eggs, for concealment, and so 
forth. In Orthezza, which has a tolerably active female, the 
male has a beautiful caudal brush. Among plants the same sort 
of thing occurs. The higher plants exhibit diverse flowers for 
recognition by insects ; but how subtle are the specific characters 
of many bacteria, fungi, and even ferns and grasses! Yet the 
species are distinct, as we see, for example, in the obviously 
different diseases produced sometimes by bacteria which are 
hardly or not-distinguishable. Thus Dr. Kanthack tells us 
(Nature, vol. lv. p. 211): ‘*No one nowadays ventures to 
define the cholera germ; there are two many varieties ofit .. . 
We have come to the conclusion that when a bacillus is morpho- 
logically identical in appearance with the diphtheria bacillus, 
and in its biological characters closely resembles the conventional 
