258 



CAUSES OF VARIATION 



Effect of heat upon growth in animals. 1 All larger land animals 

 have acquired facilities for maintaining practically a constant tem- 

 perature. This is not true for all animals, many of which, like 

 marine species, are notably dependent for their temperature upon 

 the accident of environment, in which respect they differ but little 

 from plants. It remains to note, therefore, what influence heat 

 may exert upon the growth of animal life so conditioned as to be 

 dependent upon the surroundings for its temperatures. 



INCREASE IN LENGTH (MILLIMETERS) OF YOUNG TADPOLES OF FROG AND 



OF TOAD, UNDER DIFFERENT TEMPERATURES, FROM THE 24. 



TO THE 48TH HOUR AFTER HATCHING 2 



Twenty-nine to thirty-one degrees seems to be about the 

 optimum temperature for both, from which the table shows that 

 the land-living toad prospered rather better under the higher 

 temperatures than did the frog, as he certainly suffered more 

 under the lower, but that in both the rate of growth was sub- 

 stantially in proportion to the temperature. 



The author of the table reports that the interval between 

 fertilization and hatching in cod varies from thirty days at a 

 temperature of 2, to thirteen days at a temperature of 6-8 ; 

 that herring vary from forty days at 2-4, to eleven days at 

 IO-I2; and that the time required for the frog to- attain a 

 development at which the head and tail are sharply denned is, at 

 15, six days ; at 33, one day. 



1 C. B. Davenport, Experimental Morphology, Part II, pp. 457-460. 



2 Ibid. p. 457. 



