REVIEWS. 
63 
Drysdale, and to his own researches upon the animalcules of hay-infusions, 
which in combination furnish a body of evidence against which the ad- 
vocates of abiogenesis will find it vain to contend. 
Mr. Kent further treats of the zoological affinities and distribution of the 
Infusoria, and gives directions as to the best modes of investigating and 
preserving them for examination. 
The Sponges, as already stated, are placed by the author among his 
flagellate Protozoa as an order standing next to the collared Flagellata. 
He proposes to discuss in detail the much-vexed question of the true 
position of those remarkable animals, but in this first part of his book he 
only commences the chapter dealing with it. The chief reason assigned by 
Prof. Hackel for transferring the Sponges from the Protozoa to the Metazoa 
was derived, as is well known, from their mode of reproduction, the 
* swarm-gemmules,’ or ciliated reproductive bodies of the Sponges being 
developed according to him in the same fashion as the embryos of the higher 
animal forms. Hackel’s results have not passed entirely without contra- 
diction, and Mr. Kent declares that the developmental phenomena of the 
Sponges 1 accord essentially and entirely with those presented by the typical 
Protozoa,’ and ( that there is no formation of a germinal layer or true 
tissue structure in any period of their development.’ In his next part the 
author will expound his views of sponge-nature in full detail ; and he has 
already published in anticipation two plates illustrative of the structure and 
development of Sponges. The remaining seven plates are devoted to the 
illustration of the collared Flagellata, and the figures, like all those produced 
by the author’s pencil, are exceedingly delicate and beautiful. 
If carried out in the spirit in which it has been commenced, Mr. Saville 
Kent’s Manual of the Infusoria will certainly be one of the most important 
books that we possess upon the subject of which it treats — a subject of 
interest to all philosophical naturalists, and of special interest to all the 
numerous workers with the microscope. It is to be published in six parts, 
and in the matter of execution, both as to text and illustrations, really leaves 
scarcely anything to be desired. 
While this notice was passing through the press we received the second 
part of this book, but unfortunately too late to allow us to say anything 
about it. 
THE ATOMIC THEORY* 
S INCE the early days of Greek philosophy, the hypothesis of the atomic 
constitution of matter has played no insignificant part in the history of 
physical science. It was not, however, until the beginning of the present 
century that the hypothesis took so definite a shape as to become an almost 
indispensable aid to scientific thought. To the genius of John Dalton un- 
questionably belongs the credit of having elaborated the old idea with such 
rare subtlety that the hypothesis has been elevated to the rank of a great 
* The Atomic Theory. By Ad. Wurtz. Translated by E. Clemenshaw, 
M.A. 8vo. London : Kegan Paul & Co. 1880. 
