92 ECOLOGY AND LIFE HISTORY OF THE COMMON FROG 



be density-dependent control. In industry some machines have 

 controls upon them that vary the operation of the machine according 

 to the properties of the product at the moment, but in many others, 

 it is the improbability that, say, fifty independent sources of variation 

 should all act in the same direction for long that keeps the product 

 approximately uniform. 



To quote Solomon: "There is evidence that animal populations 

 fluctuate less violently in very complex ecosystems, for example in 

 tropical forests, than in woods poorer in plant and animal species, and 

 that the most violent fluctuations tend to occur among the animals 

 in plantations of one species, or in barren places like the far north." 



Davidson and Andrewartha (1948) accounted for 78 per cent of the 

 variance in the numbers of the insect they studied by considering only 

 four weather factors. Error variances of 22 per cent commonly occur 

 in biological assays, conducted with the aid of thermostats and other 

 equipment, so that these authors are certainly right in considering 

 that they had no need to look to "density-dependent" factors to 

 explain the fluctuations of the insect. Perhaps most of the theories 

 have something right about them, for in tadpoles there are times when 

 density-dependent factors seem to come into action, but I doubt very 

 much if they are always acting. 



"And I am right. 

 And you are right, 

 And all is right as right can be." (Gilbert, 1885) 



REFEILENCES 



Balcells, E. (1956) P. Inst. Biol ApL, 24, 81-121. 



Biggs, R. and Macfarlane, R. G. (195 i) Human Blood Coagulation and its 



Disorders (BlackweU, Oxford). 

 Blair, W. N. (1927) Proc. zool. Soc. Lond., 999-1002. 

 BouLENGER, G. A. (i 897-8) The Tailless Batracians of Europe (Roy. Society, 



London). 

 Carpenter, C. C. (1953) Herpetologica, 9, 77-8. 

 Davidson, J. and Andrewartha, H. G. (1948) J. Anirn. EcoL, 17, No. 2, 



200-22. 

 Diver, C. (1933) Geog.J., 81, No. 5. 404-27. 

 Fatio V. (1872) Faune des Vertehres de la Suisse (George, Geneva). 

 FiscHER-SiGWART, H. (1897) Vjschr. naturf. Ges. Ziirich, 42, 238-316. 



