REVIEWS. 
85 
“ a. In virtue of its molecular structure, the organism may tend to vary. 
This variability may either be indefinite, or may be limited to certain direc- 
tions by intrinsic conditions. In the former case, the result of the struggle 
for existence would be the survival of the fittest among an indefinite .number 
of varieties ; in the latter case it would be the survival of the fittest among 
a certain set of varieties, the nature and number of which would be pre- 
determined by the molecular structure of the organism. 
“ b. The organism may have no intrinsic tendency to vary, but variation 
may be brought about by the influence of conditions external to it ; and, in 
this case also, the variability induced may be either indefinite or defined by 
intrinsic limitation. 
u c. The two former cases may be combined, and variation may to some 
extent depend upon intrinsic, and to some extent upon extrinsic, conditions. 
“At present it can hardly be said that such evidence as would justify the 
positive adoption of any one of these views exists.” 
With this quotation we may take leave of Professor Huxley’s book, re- 
commending it to our readers as the most moderate and philosophical exposi- 
tion of the views of the advanced school of Zoologists that they are likely to 
meet with for some time. The little volume is very freely illustrated with 
woodcuts, which, if not first-rate as works of art, generally represent well 
enough the objects they are intended to show. 
ELEMENTS OF CHEMISTRY.* 
W HEN the first edition of the Chemical Physics appeared twenty-three 
years ago, it was pointed out by the late Professor Miller that it was 
devoted to a subject upon which no elementary work had then appeared in 
this country since the publication of the excellent treatise of the late Professor 
Daniell. The interval has seen the production in an English garb, of some 
translations of text-books in which Physics has been treated from a chemical 
point of view, but the usefulness of the “ Chemical Physics ” has not waned. 
In the fifth edition, issued in 1872, so much additional matter was incorpo- 
rated that the book was half as large again as the first edition ; the present is 
nearly double its size. The main branches of the subject are, with a few 
slight modifications, treated in the same order as heretofore. In the chapter 
on the air-pump a description and woodcut are provided of the Sprengel 
pump, the complex form of which will startle a beginner ; reference is made 
in this place to the simplest form of the instrument to be found in later 
pages ; it may, however, be missed, as the number attached to the figure is 
misprinted. It is interesting, in passing, to observe that the best exhaustion 
obtained by the Sprengel pump was in one instance 9379*20000 °f the atmo- 
sphere. Allusion is made to the water barometer placed by Daniell “ in the 
hall of the Royal Society : ” we believe that an instrument of this kind, set up 
* “ Elements of Chemistry : Theoretical and Practical.” By William 
Allen Miller. Part I. Chemical Physics. Sixth Edition. Revised by 
Herbert M‘Leod. 8vo. London : Longmans. 1877. 
