( 485 ) 
However this may be, this is certain that it has been derived from 
the geological observations that in the quaternary period there must 
have been a lowering of the ground of at least 18 m. A determination 
of the duration of this period is, however, not possible. 
In virtue of this, and of a post-glacial lowering before the beginning 
of our era, which may perhaps be derived from some Mounds, we 
may state, that we are still in a period of sinking, but also that 
most probably this sinking has ceased for the last two thousand years , 
or at least been very slight during this time. 
Zoology. — “On the Age of Sainton to be Derived from the Structure 
of the Scales ”. By Dr. P. P. C. Hoek. 
(Communicated in the meeting of November 27, 1909.) 
Quite independent of human influence, salmon ascend the rivers 
they think fit in numbers which vary very much periodically. The 
influence exercised by man — be it men fishing, or men inhabiting 
and polluting the banks, or men as navigators or river engineers — 
interferes greatly with the natural production; it makes it difficult, 
in fact nearly impossible, to obtain an exact notion regarding the 
extent, the periodicity etc. of the natural fluctuations. Yet the correctness 
of the ascent varying periodically is not to be concealed by this 
♦circumstance: we see that groups of richer years alternate in a very 
striking way with groups of years of a considerable smaller catch; 
and they do so at present as well as formerly, judging from statistics 
at our disposal, say 30 or 50 years ago. 
Now seeing how the catches are composed, we soon discover that 
there is a good reason for this being the case. If we analyse the 
catch of a certain year we find that considerable differences in size 
occur between the specimens composing that catch. However, not 
all possible sizes between the smallest and the largest specimens are 
represented; on the contrary, according to their size the ascending 
salmon can be rather easily divided into three categories or size- 
groups of about the following dimensions: 
small salmon (“grilse”= Jacob-salmon) of 55—67 cm. 
middle-sized „ (small summer-salmon) » ^8 ^0 » 
and large „ (large summer-salmon and winter-salmon) „ 100—110 „ 
Based on a long experience and on a careful study of the sizes 
of the ascending -salmon, which was begun by the Basel Professor 
Mikscher Ruesch (1880) and continued by the present author (1894), 
the opinion is now well established that the smallest salmon of a 
certain year and the middle-sized 
of the 
well 
