176 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, a2 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? 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: 2s ceci deed SER eee pha TAC ENUEY ys wvenes 12h fsck ae enema: 23 
BebGianyi yc. eecleas sane aoc tae DO IIPATTETISE oy), eet th 9 eg errs em 
Marehitnsaes i223) eptember Ge So. bac oe eres iets 16 
PDT esc tsceicic <)ala ee eee oh 26) | \Octobetes jx. seca aeons 9 
Mary so 35 222 2 he ive ct eerie sealers Sep PIN Kael NS rs torino Sauces ndcan 7 
FUE): «hohe Ges eet errata len Late ik 28)\) Decembetys- sore eee 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.? 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 
2 One ot these, Calliopius leviusculus, 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. (Seep. 183, 184, below.) 
