152 PROF. E. HULL, ON THE SPREAD OF EXISTING ANIMALS 
the land connection required in the latter case occurred in the 
Pliocene and reached its culmination in the Post-Pliocene or 
Glacial period, and was, as I believe, the immediate cause of the 
occurrence of glacial conditions in the British Isles and Europe.* 
The formation of the river-valleys took place at the close of 
the Pliocene period, as proved by the late Professor Issel in the 
case of the submerged river- valleys entering the Mediterranean 
from Europe ; and the subsequent subsidence which approxi- 
mately brought about the present relations of land and sea took 
place at the close of the Post-Pliocene stage. Tims we see 
that physical changes concur with biological conditions in 
testifying to the great oscillation of level which the bed of 
the Atlantic and the adjoining lands underwent at the close 
of the Tertiary period of geological history. To us they 
seem enormous — amounting, as I have stated, to some thousands 
of feet — but as compared with the diameter of the earth they 
are comparatively insignificant, and when we recollect the 
vast changes of level which can be shown to have taken 
place in the Alps, the Pyrenees, the Himalayas and other 
mountainous regions in Tertiary times, they are not unpre- 
cedented. 
Part IY. 
PiANGE OF SOME SPECIAL ANIMAL FORMS. 
I shall now proceed to give some of the more special 
examples of animal forms common to the islands, including 
those of Great Britain, and the adjoining lands drawn from 
Dr. Scharffs work. Some of these are accompanied by artistic 
illustrations taken from life, or museum specimens, and by a 
process of shading, the areas of distribution of the animals are 
represented on a series of the map of Europe very effectively ; I 
will begin with the Auk. 
The Great and Little Auk . — In the picture! we have tire 
Great Auk standing with a whimsical air of dignity over his 
humble and diminutive companion, who is evidently quite 
content with the relative positions which nature has assigned to 
them both. Alas ! the larger of these birds has disappeared 
within the present generation, and is only to be found in our 
museums and collections. It once existed in vast numbers 
* Hull, “ Another probable cause of the Glacial period.” Trans. Viet. 
Inst., vol. xxxi. 
t European Animals, Fig 11, p. 39. 
