644 PHOTOPERIODISM IN VERTEBRATES 



erratic rainfall (Fletcher, 1889). There are fishes known to be influ- 

 enced by temperature (Burger, 1939) or suspected of being controlled 

 by the phase of the moon, perhaps as expressed through the tides. 



In this connection it is also necessary to consider the effects of yet 

 another group of external factors. While a relatively simple external 

 variant such as increasing day length or increasing temperature may be 

 all that is needed to stimulate gonad growth, it commonly happens that 

 a whole complex of suitable external factors are necessary for final 

 gonad maturation and for the production of fertile eggs. The whole 

 environment of the animal must be felt to be suitable, often a mate 

 must be present, and in some cases the social group must be properly 

 constituted. Into the first of these categories may enter the effects of 

 the weather and of the terrain, and into the second the effects of 

 appropriate courtship display and sometimes, as in the case of animals 

 like the rabbit, of copulation itself (Donovan and Harris, 1955). 

 These influences from the outside world are felt through a variety of 

 exteroceptive organs, and the resulting stimulus to the gonads is ap- 

 parently due to a final spurt of activity on the part of the pituitary 

 gland. Certainly the act of copulation is known to operate in this way 

 in the female rabbit. 



Evidently, therefore, a species may respond first to one or another of 

 a variety of environmental factors of which changing day length is one 

 example, and second to a complex of factors which may be appreciated 

 through the skin (by contact with other individuals), through the nose 

 (especially perhaps in shrews), through the ears (especially perhaps in 

 song birds), and even through the taste buds. As each new facet of the 

 necessary final "Gestalt" presents itself, so through nervous pathways 

 the pituitary activity is stimulated to new levels and the gonads pass 

 from potential to actual maturity. It may be surmised that a "good" 

 experimental animal is one to which this final complex of factors is 

 relatively unimportant, while a "poor" species is one to which it is all 

 important. 



THE NERVOUS SYSTEM AND VERTEBRATE REPRODUCTION 



Whatever may be the dominant influences entering an animal from 

 the external world, these influences must be translated into nervous 



