THE EYE IN EVOLUTION 



darkness for a month results in testicular activity, while continuous illumination 

 produces a resting state (Vaugien, 1952). The same result follows a " natural " 

 change in the day-night cycle, for if the birds in the northern hemisphere are 

 transported to the southern, their breeding season is reversed (Rowan, 1926) ; 

 while in regular migrants across the equator the stimulus for the recrudescence 

 of sexual activity and enlargement of the gonads is the shortening of the days 

 in March in southern lands (Rowan, 1938), an inherent habit which can only be 

 broken if such species are retained for several years in the southern hemisphere 

 and prevented from migrating (Marshall, 1937 ; Baker and Ranson, 1938). It 

 would therefore seem established that the sexual cycle and the migration of 

 birds, rhythins which have become innate, are determined essentially by photo- 

 period, although it is to be remembered that periods of darkness may have an 

 influence equal to or even more potent than light (Hammond, 1953 ; Kirk- 

 patrick and Leopold, 1953), while temperature also has an adjuvant effect 

 (Bissonnette, 1937 ; Farner and Mewaldt, 1952-53 ; Wilson et al., 1956). 



Similarly among mammals, male ferrets,^ mice ^ and ewes ^ can be brought 

 into oestrus in winter when normally they are in anoestrus by subjecting them to 

 rhythmic periods of increased illumination for 2 months or more, while the 

 gonads of the field-mouse have been shown to diminish by exposing the animals 

 to increased periods of darkness (Baker and Ranson, 1932).* As would be 

 expected, these changes do not apply to non-seasonable animals ^ or those that 

 reach sexual maturity dviring hibernation ® or aestivation.^ Among those animals 

 in which it is operative, however, and particularly among those with migratory 

 habits, the periodic behaviour thus induced sometimes assumes legendary 

 exactitude, a fact commented on since the days of Pliny ; the cuckoo arrives 

 in England on "Cuckoo Day", the early stream of swifts is expected to arrive 

 on the last three days of April and the big arrival on May 24th, while in the late 

 autumn each year the male markhor is said to descend from the high Hindu 

 Kush into the valleys to meet the females on December 14th precisely, and the 

 rut begins (Burton, 1951).* 



The mechanism of the action of hght in these photoperiodic 

 activities varies, but in general is mediated through hormones the 

 activity of which is largely determined by stimulation through the 

 eyes. This complex matter will be discussed subsequently,^ but at this 

 stage it is convenient to note that in Crustaceans, several hormones are 



white-crowned sparrow — Farner et al. (1953) ; dove, Zenaidura macroura — Cole (1933) ; 

 Japanese white-eye, Zosterops — Miyazaki (1934) ; duck — Benoit (1934-35), Radnot 

 (1953-55) ; quail— B. C. Clark et al. (1936-37), Hammond (1953) ; fowl— Radnot 

 (1955), Radnot and Orban (1956). 



1 Bissonnette (1932), Marshall and Bowden (1934-36), Hart (1951), Thomson 

 (1954). 



2 Whitaker (1936). 

 » Hafez (1951). 



* For further details of the mechanism involved, see p. 559. 



5 Guinea-pigs — Dempsey et al. (1933-34) ; rabbit — Smelser et al. (1934). 

 « Squirrel— Wells (1934-35), Johnson and Gann (1933). 

 ' Alexander and Bellerby (1935-38), Bellerby (1938). 



* In a similar manner the palolo (Polych^te) worms of the South Pacific shed their 

 eggs or sperms in countless millions, sufficient to give the sea the appearance of 

 vermicelli soup, at a specific time. These are eminently edible, and the natives of 

 Samoa have learned to expect a great feast precisely at dawn one week after the 

 November full moon. 



* p. 547. 



