692 The American Naturalist. [August, 
the blastomeres, more or less completely surrounding the latter, in 
which position they are finally used up as food. And the peculiar 
behavior of the follicle in Salpa is probably to be explained on the 
theory that Salpa has had an ancestor in which the follicular tissue 
persisted late in the development, and was so accurately disposed 
around and between the organs as to form what might be called a cast 
of the embryo. 
In the modern Salpa, as in the hypothetical ancestor, the follicular 
tissue develops into a cast of the embryo, but the blastomeres instead 
of leading the way as they doubtless did in the ancestral embryology, 
are now so retarded in their development that they do not begin to 
build up the embryonic organs until the follicular cast is well nigh 
completed. H. V. Wirsox. 
Bateson's Dictionary of Variaton.'—In this work the author 
has eollected a great many examples of variations from normal struc- 
tures found in animals. These include both absolute abnormalities 
and variations which are in the line of evolution. The work is a use- 
ful one to all zoologists aud students of evolution, as furnishing exam- 
ples of variation in groups with which they are not personally -famil- 
iar. It will, however, not take the place with any specialist of his 
knowledge of the subject matter of his own studies. It is not to be 
supposed that its author intended that it should. A dictionary of vari- 
ation of all animals would be a detailed work on zoology in general, 
where the normal characters of all species should be stated, in order 
that it might be shown what constitutes variation. Such a work could 
only be produced by the cooperation of a large number of “species 
naturalists.” Embryologists and histologists would be wholly unfit 
for the task. Perhaps it was a sense of this deficiency which led Mr. 
Bateson to prepare this work; for otherwise it is difficult to imagine 
why an expert in any branch of zoological sciences should attempt the 
task, unless it should be designed for amateurs and general readers. 
While preparing the work, its author neglected one of the richest 
mines of information as to normal variation. This is found in the 
writings of American specialists in vertebrate zoology, where the sub- 
ject has been treated in greater detail, and with greater wealth of ma- 
terial than exists in the literature of any other country. The book is 
well illustrated, which greatly enhances its value. We recommend it 
for study to persons who are doubtful in their opinions on the subject 
of organic evolution. 
‘Materials for the Study of Variation treated with especial Regard to Discon- 
tinuity in the Origin of Species. MacMillan & Co., London, 1894, pp. 598. 
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