BIOLOGICAL SURVEY OF WOODS HOLE AND VICINITY. 1 75 



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 Ba}- 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 groimd 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 certainly 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. 1853, p. 1451-1592; 1853. p. 153-167. 317-327. 



