156 THE ZOOLOGIST. 
one family (the Potamobiide), and those of the Southern Hemi- 
sphere to another (the Parastacide), the two groups being 
separated by the wide equatorial belt of the earth’s surface. 
Perhaps the most interesting portion of the work to the student 
as to the non-scientific reader is that comprised in the latter 
part of this chapter, wherein Prof. Huxley attempts to show how 
the theory of evolution, and that alone, can adequately explain 
the known facts relating to the morphology and distribution of 
the Crayfishes. The manner in which they may have gradually 
spread themselves over the surface of the globe is traced out, and 
it is shown how facts which at first sight would seem to militate 
against the theory of development may be satisfactorily explained ; 
how, for example, the existence of the same kind of Crayfish in 
the rivers of England and France, and the similarity of the 
Crayfishes of the Amurland and Japan, may alike be due to 
the subsidence of land that in former geological periods may 
have united what are two islands with the continents of Europe 
and Asia; while, on the other hand, the absence or scarcity* 
of Crayfishes in the area occupied by the fluviatile Crabs 
(Telphuside) may be due to the fact that the latter, being the 
stronger race, have either driven their rivals from the field, or 
successfully prevented them from entering rivers of which they 
were the earliest tenants. Not the least instructive section of 
this chapter is that devoted to the genealogy of the Crayfishes, so 
far as it can be made out from a comparative study of the fossil 
remains occurring in the various geological epochs. 
The general excellence of the woodcuts with which this work 
is illustrated, and the very complete bibliographical Index, will 
greatly assist the student who is desirous of pursuing the study of. 
the Crayfishes further into detail. We may remark, however, that 
Prof. Huxley appears to have overlooked a somewhat important 
paper by Von Martens on the classification of the Australian 
Astacide,t for he figures without certainly identifying the large 
and spiny species, which, described by Heller as A. spinifer and 
by Von Martens himself as A. arcuatus, is rightly identified by 
* Prof. Huxley does not say total absence. In Japan, for example, to cite an 
extra-European instance, Telphusa Dehaani competes with Astacus japonicus for 
the possession of the rivers, and in Australia Cheraps quadricarinatus has been 
recorded by Von Martens from Cape York, together with Telphusa transversa. 
+ Monatsber. der Akad. Wissensch. Berlin, 1868, p. 615, 
