80 
NATURE 
[ Sune 2, 1870 
analogous to the energy of position of a stone at the top 
of a cliff. 
For instance, two bodies near one another may be 
endowed with a species of energy of position due to 
opposite electrical states,in which case they have a tendency 
to rush together, just as a stone at the top of a cliff has a 
tendency to rush to the earth. If the two bodies be 
allowed to rush together this energy of position will be 
converted into that of visible motion, just as when the 
stone is allowed to drop from the cliff its energy of posi- 
tion is converted into that of visible motion. 
There is finally a species of molecular energy caused by 
chemical separation, When we carry a stone to the top of 
a cliff, we violently separate two bodies that attract one 
another, and these two bodies are the earth and the stone. 
In like manner when we decompose carbonic acid gas 
into its constituents we violently separate two bodies that 
attract one another, and these are carbon and oxygen. 
When, therefore, we have obtained in a separate state 
two bodies, the atoms of which are prepared to rush 
together and combine with one another, we have at the 
same time obtained a kind of energy of molecular posi- 
tion analogous on the small scale to the energy of a stone 
resting upon the top of a house, or on the edge of a 
cliff on the large or cosmical scale. 
BALFOUR STEWART 
FORMS OF ANIMAL LIFE 
Forms of Animal Life; being Outlines of Zoological 
Classification, based upon Anatomical Investigation, and 
illustrated by Descriptions of Specimens and of Figures. 
By George Rolleston, D.M., F-.R.S., Linacre Professor 
of Anatomy and Physiology in the University of Oxford. 
(Oxford: Macmillan and Co., 1870 ; Clarendon Press 
Series.) 
I. 
<p long-promised and hoped-for book has at last 
appeared, and we may say at once that it fully 
maintains the well-earned reputation of its learned author. 
It will probably be most useful to his own pupils, for 
whom it seems to have been originally designed ; and 
to those students of Comparative Anatomy who teach 
as weil as learn. 
The work consist of three parts : first, an enumeration 
of the anatomical characters of each sub-kingdom and 
class, arranged in a descending order from Mammalia 
to Gregarinee—a plan less useful in most respects than 
the reverse one which is now generally followed. Next 
comes a minute description of certain dissected speci- 
mens in the new Museum at Oxford ; and lastly an 
explanation of twelve plates, most of them original, which 
together supply a tolerably detailed account of the anatomy 
of at least one specimen of almost every class. “The 
distinctive character of the book consists in its attempting 
so to combine the concrete facts of ZoGtomy with the out- 
lines of systematic classification, as to enable the student 
to put them for himself into their natural relations of 
foundation and superstructure. The foundation may be 
made wider, and the superstructure may have its outlines 
not only filled up, but even considerably altered by sub- 
sequent and more extensive labours ; but the mutual re- 
lations of the one as foundation and of the other as super- 
structure, which this book particularly aims at illustrating, 
must always remain the same.” (Preface, p. vi.) This is 
very true, and it would have been well if all systems of 
classification had been thus based on anatomical facts. 
We may even suggest that the mutual relation of the 
foundation and the superstructure would have been still 
more obvious if the anatomical had preceded the systematic 
parts of the present work. 
We propose, however, to follow Professor Rolleston’s 
order, and to discuss here the classification he adopts, re- 
serving an account of the second and third parts for a 
future article. 
The question whether a perfect Zoological classification 
would be merely, like a perfect Nosology, a convenient 
method of stating and remembering a number of concomi- 
tant variations, or whether it would represent real genea- 
lorical relations between the several groups of animals, 
is one which has acquired great importance since the 
facts adduced by Darwin and Wallace have given pro- 
bability to a modified form of the old theory of evolution. 
Most German naturalists fully accept this hypothesis, and 
employ their skill in constructing genealogical trees of 
each class. Dr. Rolleston holds that acceptance or rejec- 
tion of the modern theory of evolution will depend on the 
particular constitution of each mind to which it is pre- 
sented : “but,” he adds, “whether the general theory be 
accepted as a whole or not, it must be allowed that in the 
face on the one hand of our knowledge of the greatness of 
the unlikeness which may be compatible with specific 
identity, and on the other of our ignorance of the entirety 
of the geological record, the value of the special 
‘phylogenies’ reaching far out of modern periods are 
[qy. is] likely to remain in the very highest degree arbi- 
trary and problematical.” 
It must, however, be remembered that, apart from its 
truth, a scientific theory may be very valuable by the accu- 
mulation of facts and the clearing of conceptions, to which 
it leads. Judged by this standard, the Darwinian theory 
is abundantly justified, not only by the observations of its 
illustrious author himself, but by the mass of excellent 
work it has evoked from others, especially in Germany, 
Indeed, from the results already gained, we may almost 
rank the theory of Natural Selection on a level with the 
teleological views which led Harvey to his great discovery, 
or with the belief in ideal archetypes by which Goethe was 
led to discover the presence of a pramaxillary bone in 
man, the “vertebral” construction of the skull, and the 
true morphology of a flower.* 
Again, Dr. Rolleston enumerates the many similes by 
which men have endeavoured to represent the system of 
nature, and prefers the comparison of the groups of 
animals to the islands of an archipelago. Most readers 
will probably find the metaphor of a tree with its branches 
more useful, especially if existing forms are regarded 
according to Prof. Flower’s ingenious suggestion, as the 
transverse section of such a tree, cut off at the present 
stage of the world’s history. But, after all, by far the 
most natural, convenient, and almost inevitable metaphor 
* Die Descendenztheorie wird so eine neue Periode in der Geschichte der 
vergleichenden Anatomie beginnen. Sie wird sogar einen bedeutender en 
Wendepunct bezeichnen als irgend eine Theorie in dieser Wissenschaft 
vorher vermocht hat, denn sie greift tiefer als alle jene, und es gibt 
kaum Einen Theil der Morphologie, der nicht auf's Innigste von ihr 
beriihrt wiirde. (Gegenbaur: Grundziige der Vergl. Anat. 1870, p. 19.) 
