5° Fish Cultural Association. 
mean temperature of 45 degs., a difference of more than 22 
degs. Fahrenheit.” | 
This theory would be very satisfactory if it could be ad- 
mitted that the isothermals for July indicate the actual tem- 
perature of the sea from day to day. In reality the marine 
isothermals are constantly varying, and in this respect are dif- 
ferent from those printed upon a chart, as no one knows bet- 
ter than Professor Hind. A glance at the tables appended, 
and the conclusions deduced from them in regard to the 
menhaden, will show that schools of fish do not find it neces- 
sary to force their way through walls of sea temperatures, 
but that their movements from south to north are exactly 
correlated with the seasonal rise of temperatures. As soon 
as the water at a given point reaches a certain temperature, 
which for the mackerel on our own coast appears to be as 
much as 45 degs., the fish make their appearance, and with 
the advance of the season they appear further and further to 
the north. Mackerel do not appear on the coast of Maine 
until the water there is as warm as it was off Cape Hatteras 
atthe time of their first arrival.) This is the case. whether 
we suppose their general movement to be parallel with or 
vertical to the coast line. 
I have entered the discussion of this question, not with 
any idea of attempting to prove that mackerel migrate south 
from the Gulf of St. Lawrence, but to show that a compara- 
tively rapid northward movement in May and June does not 
necessitate a “sudden plunging from high to low zones of 
temperature.” 
VI. THE ARGUMENTS FOR AND AGAINST EQUATORIAL MIGRATION. 
There is no~satisfactory evidence that the menhaden pur- 
sue extended migrations north and south. The same evidence 
