THE A:TIOLOGY OF THE CRAYFISHES. 317 
the existing American crayfishes; whether they are 
Cambant or Astact does not appear. But, in the lower 
chalk of Ochtrup, in Westphalia, and therefore in a 
marine deposit, Von der Marck and Schliiter* have 
obtained a single, somewhat imperfect, specimen of a 
crustacean, which they term Astacus politus, and which, 
singularly enough, has the divided telson found only in 
the genus Astacus. It would be very desirable to know 
more about this interesting fossil. For the present it 
affords a strong presumption that a marine Potamobine 
existed as far back as the earlier part of the cretaceous 
epoch. 
Such are the more important facts of Morphology, 
Physiology, and Distribution, which make up the sum 
of our present knowledge of the Biology of Crayfishes. 
The imperfection of that knowledge, especially as re- 
gards the relations between Morphology and Distribution, 
becomes a serious drawback when we attack the final 
problem of Biology, which is to find out why animals 
of such structure and active powers, and so localized, 
exist ? 
It would appear difficult to frame more than two 
fundamental hypotheses in attempting to solve this pro- 
blem. Hither we must seek the origin of crayfishes in 
conditions extraneous to the ordinary course of natural 
* Neue Fische und Krebse aus der Kreide von Westphalen. Palzon- 
tographica, Bd, XV., p. 302; tab. XLIV., figs. 4 and 6. 
