IJ$ BULLETIN OF THE BUREAU OF FISHERIES. 



the season of growth and reproduction" (1895, p. 233); while "animals and plants are 

 restricted in southward distribution by the mean temperature of a brief period covering 

 the hottest part of the year" (p. 234). The phrase "total quantity of heat" above 

 employed is not to be taken in a strict sense, however, but implies "the effective tem- 

 peratures or degrees of normal mean daily heat in excess of this minimum [6° C.]" 

 which " have been added together for each station, beginning when the normal mean daily 

 temperature rises higher than 6° C. in spring and continuing until it falls to the same 

 point at the end of the season" (p. 232-233). "In conformity with the usage of bota- 

 nists, . a minimum temperature of 6° C. (43 F.) has been assumed as marking the 

 inception of the period of physiological activity in plants and of reproductive activity 

 in animals" (p. 232). 



It is obviously impossible without qualification to apply this principle in explaining 

 the distribution of marine animals. Many of these, as is well known, breed during the 

 coldest months of the year, at a time when the temperature lies considerably below that 

 assumed by Merriam as a necessary minimum for physiological activity ; and there is no 

 general agreement in the breeding season of even closely related forms. Unfortunately, 

 the period of sexual reproduction is not definitely known for the vast majority of our local 

 species. The greater part of such definite observations as are available are contained 

 in the rather meager notes of Bumpus (1898, 1898a, 1898b), Mead (1898), and Thomp- 

 son (1899), which cover only the spring and summer months. For a few species, how- 

 ever (e. g., certain amphipods a and the mollusk Littorina palliata), we have definite 

 evidence that eggs are laid nearly or quite throughout the year. 



From the data offered by Garstang (1894) for the breeding periods of marine 

 animals at Plymouth, England, we may make a rough computation of the percentage 

 of the species which breed during each month of the year in those waters. The follow- 

 ing table, based upon records for about 200 species, presents these figures: 



Per cent. Per cent. 



January 14 



February 20 



March 23 



July 23 



August 21 



September 16 



April 29 October 9 



May 33 November 7 



June 28 I December 5 



It is impossible to state how far these figures are representative of the total marine 

 fauna, even at Plymouth, and how far they have depended upon the relative activity 

 of the observers during different months, but they seem to show that a considerable 

 proportion of the species reproduce during the coldest months of the year. 6 And it 

 would be a safe assumption, even in the absence of such confirmatory evidence as we 

 possess, that the same statement would hold for the region of Woods Hole. 



Before the operation of Merriam's law can be accepted as a sufficient explanation 

 of the non-occurrence of certain southern species in the colder waters of this region, it 

 must be shown that the "season of growth and reproduction" coincides with the period 

 during which these waters are colder. As a matter of fact, we know that a considerable 



1 One of these, Calliopius leeviusculus, is included in the list of the cold-water, northward-ranging species, which are, in our 

 dredging records, restricted to the western part of Vineyard Sound. 



b It must be added, however, that the waters in the neighborhood of Plymouth never reach such low temperatures as are 

 recorded during the winter months for Vineyard Sound and Buzzards Bay. (See p. 183, 184, below.) 



