448 AMERICAN FISHES. 
Merrimac in May and June, in the Penobscot most abundantly in June 
and July, though some come as early as April, and in the Miramichi, from 
the middle of June to October. I can only account for this seeming 
paradox by the theory that, while Salmon are not harmed by extreme 
variation of temperature, they may be averse to sudden changes, and 
though strongly impelled to seek the spawning grounds are prevented by 
the cold. I have ascertained that the cod possesses very little animal 
warmth. The temperature of the blood of a number of individuals caught 
in twenty-five fathoms of water was 47’, F., precisely that of the water 
at the bottom whence they were lifted. Mackerel swimming at the sur- 
face registered 59° or 60°, while the temperature of the water was 58°, 
thus indicating that they possess a trifling amount of animal heat. The 
Salmon unquestionably changes its temperature with that of the surround- 
ing water in much the same way, and if, as is probable, rivers rising in the 
mountains are colder in early spring than the ocean strata frequented by 
the Salmon, here is a possible solution of the problem. It is stated that 
in the English rivers, which are always open, there are no regular seasons 
of ascent, the fish constantly passing in and out; indeed, Mr. Atkins 
thinks it pretty certain that large Salmon in prime condition are running 
into the Penobscot from the sea every month in the year. It is likely, 
also, that the warmth of the rivers is an important factor in accelerating 
the vegetative growth of the eggs in the ovaries of the mother fish. 
The movements of the Salmon are not so intimately related to the tem- 
perature of the water as those of many other species. They are not 
sensitive to sudden changes, and are capable of enduring a range of at 
least forty-five degrees. In this they resemble less the migratory fishes 
than the permanent residents of our fresh waters; indeed, it is quite 
allowable to speak of them as resident, for a large proportion of the 
whole colony belonging in one river may be found in it at any season. 
This proportion cannot fall much below two-thirds, if we consider that 
the fish less than a year old would make up at least half its number, and 
that the breeding fish are in the rivers six or seven months after the breed- 
ing. The breeding fish remain during the season_of greatest heat and 
greatest cold, though their stay after they have deposited their eggs is no 
doubt chiefly because their vitality is diminished and their circulation 
retarded by the falling temperature, depriving them alike of the craving 
for food and the power to seek it. Those which spawn early are believed 
