BIOLOGICAL SURVEY OF WOODS HOLE AND VICINITY. 175 
Although we believe these evidences of the distribution of local species with relation 
to temperature to be well-nigh conclusive, the fact must be admitted that there occur 
in Vineyard Sound and Buzzards Bay a considerable number of predominantly north- 
ward-ranging species, and a yet greater number of southward-ranging ones, whose 
distribution within local waters bears no possible relation to temperature. These are 
in some cases of very general occurrence; in others their distribution appears to be 
determined by the character of the bottom. 
The actual mode of operation of temperature in restricting the distribution of 
species locally is not easy to state, and it is probable that no single formula is appli- 
cable to all cases. In chapter 11 the temperature conditions throughout local waters 
have been discussed rather fully. It has been shown that the temperature of those 
portions of Vineyard Sound and Buzzards Bay which immediately join the ocean is 
lower than that of the more inclosed waters for probably not more than half of the 
year, the difference being greatest during the summer months. It was also shown to 
be probable that all the waters of the region reach a point not far from the freezing 
point of salt water for a longer or shorter period during the winter. In the light of 
what we know regarding local temperature conditions on the one hand and the dis- 
tribution of our marine fauna on the other, it will be of interest to consider certain 
theories which have been put forward to explain the part played by this factor in 
limiting the distribution of organisms in general. 
The influence of temperature in determining the distribution of marine animals 
was emphasized by Forbes and by Dana more than 50 years ago, and has been accepted 
as almost self-evident by a large number of naturalists. Just how this factor operates 
in limiting the distribution of a given species is, however, far from plain. Dana,® in 
1852, introduced the concept of ‘‘isocrymal lines,’’ or lines showing the mean tem- 
perature of the waters along their course for the coldest 30 consecutive days of the 
year. Ordinary isotherms, or lines of mean annual temperature, he rejects as inade- 
quate, on the ground that ‘“‘the cause which limits the distribution of species north- 
ward or southward from the Equator is the cold of winter rather than the heat of 
summer or even the mean temperature of the year’”’ (p. 1452). 
Such a principle cértainly does not explain the effect of temperature upon distri- 
bution within the limits of our local waters. Here the minimum winter temperatures 
are probably nearly the same throughout the entire region. If there are any local 
differences of regular occurrence, it is without doubt the shallower, more inclosed 
waters which attain the lowest winter temperatures. But these are precisely not the 
ones which are occupied by the northern forms of which we have spoken. Within 
local waters it is certainly the summer temperatures rather than the winter ones 
which are chiefly effective in limiting the distribution of species. 
Verrill (1866, p. 249) maintained that for birds ‘‘the essential limiting cause is 
the average temperature of the breeding season, which for the majority of our birds may 
be taken as April, May, and June.’’ This idea was apparently suggested by the con- 
clusions of certain botanists respecting the distribution of plants. Merriam (1895, 
1898), following out the same thought, has been led to the belief that ‘‘[land] animals 
and plants are restricted in northward distribution by the total quantity of heat during 
@ Dana, 1852, Pp. 1451-1592; 1853, P. 153-167, 317-327. 
