Amphibia 107 



inconceivable why they should now be restricted to that 

 eastern corner, while there are hundreds of other suitable 

 places, which, if on the Continent, would be perfect paradises 

 for Water-frogs. 



However, we know next to nothing about the oecology of 

 these creatures. It is quite possible that the sporadic occur- 

 rence of the 'car. lessonae is due to local adaptation and 

 changes of the typical form, wherever the same favorable, or 

 necessary, but to us unknown, conditions prevail. And after 

 all, the differences between these two varieties are not great, 

 and in many specimens are even arbitrary, just as we might 

 expect in actually changing forms. On the other hand, we 

 know of a good many species which are either actually spread- 

 ing, or which and this may apply to the present case are 

 slowly but surely vanishing. In prehistoric times Emys 

 europaea, the Pond-tortoise, was common in the Fens. It has 

 now receded eastwards, being extinct in Belgium, Holland 

 and west of the Elbe; between this river and the Oder it is 

 now in the vanishing stages, while it is still plentiful in 

 Poland and Russia. Tempora mutantur, nos et mutamur in 



