VERTEBRATE PHOTOSTIMULATION 643 



tioned above. However, the evidence regarding autumn-breeding 

 species is clear. From the early studies of Hoover and Hubbard (1937) 

 on the brook trout, of Bissonnette (1941) on the goat, and of Yeates 

 (1947) on the sheep, the conclusion emerges that in at least some 

 autumn-breeding species a shortening day, or a lengthening night, is a 

 critical factor in the stimulation of gametogenesis. 



This conclusion is of great theoretical importance, showing as it does 

 that a species may come to possess an annual breeding cycle which is 

 attuned either to the increasing day length of spring or to the decreas- 

 ing day length of autumn. It follows that the secretion of gonadotropins 

 from the pituitary is not the necessary consequence of increasing day 

 length as such, and that, because of some special need, a species may, 

 through natural selection, be adjusted to receive the stimulus to game- 

 togenesis from whatever kind of day length is typical of the most suit- 

 able time of the year. 



OTHER EXTERNAL FACTORS AND VERTEBRATE REPRODUCTION 



It is also evident that in many species, such as those which are 

 subterranean, it cannot be anticipated that light will have any effect at 

 all. Little or nothing is known of the methods of control of the repro- 

 ductive cycles of such animals, but it has already been proved that 

 some of the cold-blooded vertebrates, such as Rana temporaria (van 

 Oordt and van Oordt, 1955), react not to changing light but to 

 changing temperature. 



Unfortunately, relatively little effort has been devoted to the study 

 of vertebrates whose reproductive cycles are apparently controlled by 

 factors other than light. There are many known examples of such 

 vertebrates, and our ignorance of this matter is serious. There is the 

 tropical bat described by Baker and Bird (1936), which is exposed to 

 an almost unchanging length of night, which roosts in an almost 

 thermostatic cave, but which has a sharply defined annual breeding 

 season. There are many tropical birds which may breed in time to the 

 onset of the rainy season (Marshall and Disney, 1957). There are frogs 

 whose spawning period is controlled by temperature (van Oordt and 

 van Oordt, 1955) or which live in deserts and breed in time to an 



