chapter 8 



THE INFLUENCE OF EXTERNAL FACTORS ON 

 THE SPAWN DATE 



In dealing with the relation between the spawn date and the environ- 

 ment we enter a field that is bound to be compHcated. It is my 

 task to present this to the reader as clearly as I can, but, if I am to 

 do justice to the subject, I cannot simplify it by leaving out the com- 

 phcated parts. We cannot appreciate delicate detail while we are 

 walking rapidly past it, and no picture of it painted in a few bold 

 strokes can possibly convey what is there. My role, then, is to act 

 in the way that a spectator who has been there before would do to a 

 stranger, and to point out the intricate tracery of a frog's hfe, inter- 

 woven with the environment, for that surely is what will appear in 

 the following pages. 



It is an illusion to consider that a statistical treatment of this subject 

 is optional. No human mind can disentangle such a web of interacting 

 factors without the aid of statistics, and the obscurity surrounding 

 the subject until these methods were apphed was due undoubtedly to 

 attempting the impossible. In this chapter, therefore, I include as 

 much statistical matter as I think necessary to explain what has been 

 done, but consign technical details to Appendix i h, in. Chapter lo. 



The breeding cycle of Amphibia, as in other animals, is under the 

 immediate control of hormones, which are themselves regulated by 

 those of the pituitary. In choosing for the title of this chapter a phrase 

 that included the word "external," I deliberately excluded such 

 influences. In my view, they are the conducting mechanisms, the 

 telephone apparatus, but not themselves the autonomous initiators of 

 breeding behaviour. Beach (1948) has stated the matter thus: "Hor- 

 mones may be considered as internal stimuli or as part of the internal 

 environment. In either case they are seen to be only one of several 

 factors that operate in conjunction to produce a particular pattern of 

 behaviour. In the absence of additional influences, particularly in the 

 case of stimuh of external origin, hormones are powerless to affect 

 behaviour." Six years later. Smith (1955), in a Symposium on the 

 Comparative Endocrinology of Vertebrates, said in his paper on 



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