124 



ANALYSIS OF THE ENVIRONMENT 



garis. The speed of reaction of the testes is 

 increased with higher intensities. 



With the starling, red light is more ef- 

 fective than white and green in stimulating 

 gonadal activity, and green light may actu- 

 ally be inhibiting when the relative heat 

 energy reaching the birds is 10:1:2.5 for the 

 tliree types of light (Bissonnette and M'^ad- 

 lund, 1931, 1932). Supporting evidence of 

 the differential effects of different wave- 

 lengths on the breeding cycle is found 

 among mammals. The winter anoestrus 

 period of ferrets can be broken by increas- 

 ing the length of day. The activating radia- 

 tion extends from the red (6500 A) to the 

 near ultraviolet (3640 A). The fairly sharp 

 threshold at the red end, 7500 A, is barely 

 effective even when its intensity is high; 

 this indicates that the effect is produced by 

 visible light rather than by heat. 



An interesting geographical experiment is 

 furnished by the shifting of animals from 

 the northern to the southern hemisphere, or 

 vice versa. When ferrets are transferred 

 from the north during the period of length- 

 ening or long days in spring or summer to 

 the similar period well south of the equator, 

 a change-over occurs in their breeding pe- 

 riod corresponding to that induced by a 

 comparable experimental change in the 

 length of day among laboratory ferrets in 

 the north. This tendency is not shown by 

 bird migrants that penetrate deep into the 

 southern hemisphere during our northern 

 winter (the golden plover is an example). 

 For such birds the annual rhythm may have 

 become sufficiently stabilized so that it is 

 not susceptible to alternation by the ex- 

 posure gained in a single season. There is 

 some evidence that storks, if held well south 

 of the equator, will in time adopt the rhy- 

 thm imposed by the southern environment. 

 Deciduous trees transplanted into southern 

 latitudes may show a similar lag of a few 

 years before they become adapted to the 

 changed conditions (Bissonnette, 1935; Ro- 

 wan,' 1931). 



PENETRATION OF LIGHT INTO WATER 



Most solid objects in nature are opaque 

 to light— ice is an exception. The transpar- 

 ency of water is affected by many factors, 

 among which are the following: (1) angle 

 of incidence of the sun's rays that varies 

 with the time of day, the season of the year 

 (except at low latitudes), and latitude 

 itself; (2) reflection from the surface (this 



is related to the angle of incidence, and re- 

 flection increases when the surface is ruf- 

 fled, as it usually is); (3) thickness of the 

 layer through which the light must pass; 

 (4) clearness of the water as regards color 

 and turbidity; (5) the wavelength of the 

 light; and (6) the intensity of the incident 

 light. 



Measurements summarized by Welch 

 (1935, p. 75) indicate that, in midnorthern 

 latitudes, surface loss may run from 5 to 70 

 per cent. Of the light that enters the water, 

 about one-third is lost in the first meter, 

 about three-fourths in the upper 5 meters, 

 and only about one-tenth remains at 10 

 meters' depth. These figures give orders of 

 magnitude for relatively clear salt or fresh 

 waters. From the surface downwards, light 

 intensity is reduced according to the follow- 

 ing equation (Clarke, 1939): 



I 



r "^' 



where L is the initial intensity; I is the 

 final intensity; k is the coefficient of extinc- 

 tion; L is the depth in meters, corrected to 

 give the mean path of the light, since this is 

 usually greater than the vertical depth; and 

 e is 2.7. When reduction of light intensity 

 and water depth are plotted semilogarith- 

 mically a straight line is obtained, the slope 

 of which is detennined by the extinction 

 coefficient k, which becomes an index of 

 transparency. 



Light may penetrate a thousand meters 

 or more in the open, subtropical ocean; 

 photographic plates are darkened at 1500 

 meters in mid-Atlantic. At a thousand 

 meters the amount of light is reduced to 

 3 X 10"° of that 1 meter below the surface. 

 The euphotic stratum in the open ocean 

 reaches a depth roughly of 80 meters (30 to 

 100 meters) and is succeeded by a dyspho- 

 tic stratum, sometimes called the twilight 

 zone, that extends to the effective limit of 

 light, a limit that often occurs at some 200 

 to 600 meters. On an exceedingly brilliant 

 day, Beebe, in a "bathysphere dive," found 

 light still visible to his eye at 571 meters; 

 at 610 meters all visible daylight had van- 

 ished. In an earlier descent he found the 

 lower limit of visibly detected light at 511 

 meters (Beebe, 1932, 1934). 



The higher the latitude, the narrower is 

 the lighted stratum, and marine organisms, 

 accordingly, are more concentrated near the 



