228 THE RATE OF GROWTH [ch. 



That is to say, between the intervals of 10° and 20° C, if it take 

 m days, at a certain given temperature, for a certain stage of 

 development to be attained, it will take m x 1-128" days, when the 

 temperature is n degrees less, for the same stage to be arrived at. 



Fig. 66 is calculated throughout from this value; and it will be 

 found extremely concordant with the original diagram, as regards 

 all the stages of development and the whole range of temperatures 

 shewn; in spite of the fact that the coefficient on which it is based 

 was derived by an easy method from a very few points on the 

 original curves. In hke manner, the following table shews the 

 "incubation period" for trout-eggs, or interval between fertihsation 

 and hatching, at different temperatures * : 



Incubation-period of trout-eggs 



Choosing at random a pair of observations, viz. at 3-6° and 10°, 

 and proceeding as before, we have 



10° - 3-6° = 64°. 



Then (64) = '-g, 



or 6-4 X log X = log 135 — log 47 



= 2-1303 - 1-6721 = 0-4582 

 and log X = 0-4582 - 6-4 = 0-0716, 



X = 1-179. 



* Data from James Gray, The growth of fish, Journ. Exper. Biology, vi, p. 126, 

 1928. 



