THE ENVIRONMENT AND REPRODUCTION 25 



widely from year to year, but in a particular latitude a 

 given day always has a given length which is longer by a 

 given interval than the day just past. 



If this reason were not sufficient for suspecting that 

 daylength might be a critical external factor in control- 

 ling seasonal reproduction, then it could have been noted 

 that in some parts of the world poultry farmers had for 

 a long time been using artificial light in autumn and 

 winter in order to stimulate unseasonal egg production. 12 

 A similar practice was common in Japan whereby from 

 ancient times pet birds, such as Zosterops, were brought 

 into spring song in January. The method, known as 

 'yogai', was achieved by exposing the birds in autumn to 

 some 3 or 4 hours of artificial light after sunset. This 

 again is similar to an old Dutch custom, recalled by van 

 Oordt, whereby many songbirds were caught in autumn 

 with singing decoys. These decoys were put into the 

 dark in May and exposed to light again in August so 

 that by September and October when they were used, 

 they were in full song as in the breeding season. 



An analysis of the effect of light on breeding is given 

 in the following section, and the efl^ects of other external 

 factors are considered on p. 36. However, before 

 proceeding, it is necessary to reinforce Baker's warning^^ 

 against the misuse of the term 'exteroceptive' in the 

 literature of this subject. The expression 'exteroceptive 

 factor' has been widely used in preference to t"he ex- 

 pression 'external factor'. The word exteroceptive was 

 originally coined by Sherrington who used it, in contrast 

 to proprioceptive, to mean 'receptive of external stimuli'. 

 Thus the eye may be properly called an exteroceptive 

 organ, but the light which it appreciates is clearly an 

 external and not an exteroceptive factor. 



4. THE EFFECTS OF LIGHT 



Although practical men, by rule of thumb, may have 

 discovered long ago the magical influence of light on 



c 



