282 Reviews—Prof. Dr. M. Neumayr— 
this period more than thirty distinct zones, each with its own 
peculiar fauna, can be distinguished; and making sufficient allow- 
ance for forms common to more than a single zone, the number of 
species which may have existed during the whole Jurassic period 
can hardly have been fewer than from 500,000 to 750,000. And 
yet the total number of marine fossil species from all the Jurassic 
beds only mount up to about 10,000, a very insignificant pro- 
portion in comparison with the above estimate. If such deficiencies 
exist where the geological series is fairly continuous, still greater 
ones may be presumed where there are breaks in the succession like 
those between the Tertiary and Mesozoic, and the Mesozoic and 
Paleeozoic groups. 
In view of the above it is useless to expect that a complete 
succession of fossils could be found in which all the varieties inter- 
mediate between different species were represented ; but nevertheless 
fairly distinct lines of descent can be shown in the case of certain 
Brachiopods, Corals and Crinoids from the Paleozoic strata; in the 
Mesozoic Ammonites and the Molluscan genera Pholadomya, Inoce- 
ramus and Halobia, and in many Tertiary forms. As good examples 
of a gradually changing series of forms may be cited the fresh-water 
Paludinas from the Lower Pliocene of Sclavonia, in which the 
individuals at the beginning vary so much from those of the higher 
end of the series, that they might be taken for distinct genera if it 
were not for the numerous intermediate connecting forms. 
The causes which have led to the extinction of species are to the 
paleontologist of as great interest as those relating to their origin. 
A great number of species have become extinct in historical times, 
some mainly through man’s influence. It can hardly be said that 
species become extinct because they have outlived their capacity to 
vary, for we find still existing genera, such as Rhynchonella, Lingula, 
and Discina, which have varied but very little since their first appear- 
ance in the Paleozoic era; whilst on the other hand such forms as 
Ammonites and Rudistes exhibit remarkable variations shortly before 
they became extinct. The complete extinction of many groups of 
organisms which existed through long geological periods in great 
numbers and in manifold variety, as, for instance, the Trilobites, is 
difficult to account for; but there is nothing to support the theory 
that this or any other group of animals inhabiting widely-separated 
localities became contemporaneously extinct ; for so far as it can be 
traced, the extinction of a species is not sudden, but rather a gradual 
process. In certain instances it is possible to connect the disappear- 
ance of a predominant group with the gradual increase and sub- 
sequent supremacy of another group which thus supplants it, and 
furnishes an instance of the ‘survival of the fittest’; but in other 
cases, such as the Carboniferous Fusulina and the Tertiary Num- 
mulites, there is no evidence of extinction from such a cause. 
The next five chapters of the work are devoted to the considera- 
tion of the following invertebrate groups: I. Protozoa; I. Coelen- 
terata; III. Echinodermata; IV. Annelida; and V. Molluscoidea. 
As a preliminary introduction to each group, the most important of 
its structural details are given with figures of the typical forms. 
