LIGHT AND METABOLISM 17 



at different periods of the daily cycle. In snails, a prolongation of the diurnal 

 period of light beyond 13 hours stimulates egg-laying, while periods of 11 hours 

 or less inhibit it (Jenner, 1951) ; it is interesting that short intervals of illumina- 

 tion during the dark periods of a short-day cycle stimulate egg-laying, showing 

 that, as with short -day plants, the length of the dark period is an essential 

 feature in the stimulus. Similarly the strawberry -root louse. Aphis forbesi, can 

 be made to breed in midsummer instead of February by artificially curtailing 

 the summer days (Marcovitch, 1923). A more dramatic influence is seen in the 

 plant-louse, Psylla : individuals hatched in autumn differ from those hatched 

 in spring but the winter-type can be produced in spring by subjecting the larv£e 

 to an artificial diurnal rhythm in which the period of light is shortened (Bonne- 

 maison and Missonnier, 1955). 



Among FISHES, temperature has been shown to be a potent factor, but it 

 has been demonstrated that the reproductive cycle of the trout can be photo- 

 periodically determined (Hoover and Hubbard, 1937) ; similarly the activity 

 of the gonads of certain amphibians such as the clawed toad, Xenopus Icevis, 

 (Shapiro and Shapiro, 1934) and reptiles such as the lizard, Anolis carolinensis 

 (Clausen and Poris, 1937), has been altered by means of artificial illumination. 



BIRDS show more dramatic changes than most species, and these have 

 received much attention probably because of their obvious habits of migration 

 and singing, the sexual connection of which has been recognized since the time 

 of ArLstotle. For long the annual rhythm of the avian gonad was held to be 

 determined bj^ temperature. It is true that in the old custom of " muit " long 

 prevalent in Holland, birds were brought into song in autumn by confining them 

 in the dark in the middle of June and exposing them to light in September, and 

 that by the similar ancient practice of "yogai", Japanese pet birds were brought 

 into singing condition in January by providing them with extra hours of 

 illumination in the autumn (Miyazaki, 1934). In this respect, however, zoologists 

 waited on botanists ; for although Schafer (1907) had suggested that migration, 

 because of its accurate periodicity, must depend on the mathematically regular 

 changes in length of day rather than on the notoriously irregular variations in 

 climate, it was not until the work of C4arner and AUard on the influence of photo- 

 periodism on the flowering of plants had been published in 1920 ^ that Eifrig 

 (1924) propounded a similar hypothesis to explain the habits of birds, a con- 

 ception eventually proved by the experimental work of Rowan (1925-38). 



Rowan's classical work was on the junco finch, which migrates from wintering 

 grounds in the middle United States to Alberta ; he found that even if the birds 

 were retained in an aviary in Alberta, provided they were subjected to the 

 artificial increase of daily illumination (2-3 mins.) that they would have 

 experienced in the early spring in the States, their gonads matured and they 

 bvirst into song in December despite the temperature of the Canadian winter 

 (minimum, — 50^F). These results were confirmed by Bissonnette (1930-32) 

 in Connecticut experimenting on starlings ; and it is now amply established 

 that among many birds of the temperate zones of the northern hemisphere, the 

 testes of which normally reach a peak of activity as the days lengthen in late . 

 spring, an artificial increase of the period of illumination over some time brings 

 on a precociovis activity, while a curtailment or denial of light brings on the 

 reverse changes.- On the other hand, confinement of male parrots in continuous 



1 p. 10. 



2 See among others : junco, Junco hyemalis — Rowan (1929), Jenner and Engels 

 (1952) ; starling, Slurnus vulgaris — Bissonnette (1930-32) ; pheasant — Martin (1935), 

 B. C. Clark et al. (1936-37) ; house span-ow. Passer domesticus — Riley (1936), Kirsch- 

 baum and Ringoen (1936) ; white-throated sparrow — Jenner and Engels (1952) ; 



S.O. — VOL. I. 2 



