481 
to the ends of the months it will be possible, for the purpose of 
utilizing our data in monthly totals and means, to divide the 
year into two periods, April-September and October—March, of 
greater and less illumination respectively. The contrast in 
illumination is further heightened by the fact—derived from the 
following table of cloudy days—that the number (at Havana, 
159) of cloudy days between the vernal and autumnal equi- 
noxes is only about one half that (311) between the autumnal 
and vernal. On the average, the season of greatest light is also 
the season of greatest production. Thus, in the channel waters 
average monthly production in April-September ( 4.76) is seven- 
fold that in October—March (.67), and in the backwaters, such 
as Quiver, Thompson’s,and Phelps lakes,it is respectively 5-, 
2.2-, and 1.6-fold greater. The records of individual years in 
all of the localities will be found to exhibit a similar relation- 
ship. We may infer, accordingly, that the increased light be- 
tween the vernal and autumnal equinoxes tends to increase 
production, and that the decreased amount in the remainder of 
the year tends to lower it. It operates, of course, in conjunc- 
tion with other factors, and our records contain not a few in- 
stances where production in the period of less illumination ex- 
ceeds that in the period of greater light. For example, on 
December 20, 1898, in the minimum illumination of the year 
and under an ice-sheet 21 em. thick, which still further reduced 
the light, the plankton production in Thompson’s Lake reached 
an amplitude (2.58) exceeding that on June 21 (2.47) in the 
same lake in maximum illumination,an amplitude, moreover, sur- 
passed but once from June to October. Other factors are thus, at 
times, at least, more potent than light in controlling production. 
A phenomenon of like import exists in the conditions of 
illumination and production in Quiver and Thompson’s lakes. 
Both lakes are of approximately the same depth, but the 
former, especially in low-water conditions, has remarkably clear 
water, the bottom being generally visible, while the latter is 
always more turbid, and light penetrates the water far less 
completely. Nevertheless, the lake, with most illumination, 
