380 JOURNAL OF SCIENCE. 
of Newton, because ‘it is founded upon a single law, a single effective 
cause, namely, upon the interaction of adaptation and _ heredity.’ 
They overlook the fact that we have here only to do with the proof of 
a mechanical and causal connection between series of biological 
phenomena, and not in the remotest degree with a physical explana- 
tion. Even if we are justified in connecting the phenomena of 
adaptation with the processes of nourishment, and in conceiving 
heredity as a physiological function of the organism, we still stand 
and regard these phenomena as ‘the savage who sees a ship fur the 
first time.’ While the complicated phenomena of heredity remain 
completely unintelligible, we are only in a position to explain in 
general terms certain modifications of organs, on physical grounds, 
by the altered conditions of metabolism. It is only rarely—as in the 
case of the operation of use or disuse—that we are able more directly 
to relate the development or the atrophy of organs to the increase or 
decrease in their nutrient activity, z.e, to give a chemico-physical 
explanation.” 
The following passage also contains a salutary warning; although 
we cannot but regret that Prof. Claus has not availed himself of 
diagrams—whether calling them phylogenies or not—to illustrate the 
present state of knowledge as to the relations of the various animal 
eroups. “Only the ultimate twigs of the enormously ramified ances- 
tral tree are accessible to us in sufficient number. Only the extreme 
points of the twigs are completely preserved; while of the numerous 
ramifications of the branches only the existence of a stump here and 
there has been demonstrated. Hence it appears quite impossible, in 
the present state of our knowledge, to attain to a sufficiently sure 
representation of this natural genealogical tree of organisms: and 
while we admire the bold speculations of E. Haeckel’s genealogical 
attempts, it must be admitted that at present there is room for 
innumerable possibilities in detail, and that subjective judgement holds 
a more conspicuous place than objective certainty of fact. Hence we 
must be contented for the present with an incomplete and more or less 
artificial arrangement; although the conception of the natural system 
theoretically is established.” 
We miss one or two matters of considerable importance from this 
otherwise excellent Introduction : there is no account of the pheno- 
mena of symbiosis; of the nature and function of animal chlorophyll ; 
or of the diffused condition of the nervous system which has recently 
been found to characterize so many of the lower types. 
In the Special Part a uniform mode of treatment is observed. 
The general characters of the “Type” are first given, then those of 
the classes and sub-classes included therein, each group being headed 
by a brief definition in italics; the chief characters of the families 
with the names of the principal genera, are given in small type. This 
systematic arrangement facilitates reference and gives the student a 
clear idea of the nature and value of the various groups. 
The classification is little altered from that adopted in the Third 
Edition of the author’s “Grundziige,”’ published in 1876. The 
Schizomycetide (Bacteria) and the Gregarinide are still most un- 
accountably placed in a smalltype Appendix to the chapter on 
Protozoa. Far more serious is the omission of any account of the 
Dicyemide and Orthonectide which are only referred to in a short 
