1908.] NATURAL SCIENCES OF PHILADELPHIA. 437 



Table XIV — Effect of Heat and Cold. 



5. Lt^/i^.— Beginning with Higgenbottom ('50), various writers have 

 conducted experiments on the effect of Hght on the growth of animals. 

 The work of these authors, including Yung (78, '80 and '92), Vernon 

 ('95), Warren ('00) and Beclard ('58), deals largely with the effect of 

 colored light on the development of different animals. The present 

 work considers the effect of hght and darkness alone. Although 

 experiments were attempted with colored lights, yet the many difficul- 

 ties in the shape of uncontrollable factors made the results so unreliable 

 that space will not be taken in discussing them. Even in the experi- 

 ments on light and darkness the factor of food was with difficulty con- 

 trolled. It was only by resorting to similar means as in the experi- 

 ments on temperature that this factor was controlled at all. However, 

 this did not remove all the uncontrolled factors present. There was 

 also the chance of there being a different temperature of the water 

 between the two jars; this difference at times amounting to 2° C. 



