193 
for breeding, while their eggs are deposited prior to their 
_ leaving, so whether they come to perpetuate their race or seek 
food for preventing death in each individual of the species, it 
eventuates that at these periods breeding usually occurs, as 
may be observed in the herring or in the mackerel. It has 
been asserted that anadromous forms, or such as live in the 
_ sea but deposit their eggs and mostly rear their young in 
_ rivers, do not enter these solely for breeding purposes, but that, 
irritated beyond endurance by some marine parasites, they 
ascend into fresh waters in order to rid themselves of their 
tormentors. And in like manner that they leave rivers in 
_ order to cause the death of certain fresh-water forms, which 
give them no rest. Putting aside these far-fetched and 
theoretical reasons for migration, there is certainly one which 
is the chief cause or the necessity for the continuation of the 
_ species. For this purpose several anadromous forms pass up 
_ rivers, sometimes for long distances, and then deposit their 
ova. Among the most widely distributed of such is the shad, 
_ of which we possess two species in the British Isles, both found 
in the Severn, but up which they now but rarely ascend in 
numbers to any considerable extent, due to weirs across the 
‘ river, deficiency of water, or else its poisonous condition. The 
salmon similarly ascends from the sea to deposit its ova in 
rivers and streams, and this instinct of migration or necessity 
: p for exchanging its locality to a suitable breeding spot may be 
more or less observed among our trout, charr, and, in fact, all 
A the Salmonide. 
Even some fresh water forms show migratory propensities 
_at the breeding season, but which have mainly to be looked for 
in larger countries than ours, and where fishes may be observed 
‘upon a more extended scale. The mahaseer of India is a barbel 
(Barbus,) but in the east it does not deposit its ova, as a rule, 
ir the rivers of the plains, but migrates during the rains up 
‘such as descend from Alpine origin, when turning into a side 
stream it forms its nursery, having done which it rapidly 
descends, in order to obviate being cut off by falling waters. 
Here the young have the head waters to themselves until 
