404 THE STUDY OF ANIMAL LIFE CHAP. 



is a mutant stock that produces supernumerary legs, a 

 considerable percentage in winter, few or none in summer. 

 Miss Hoge finds that when the flies are kept in an ice- 

 chest at a temperature of about 10 C., a high percentage 

 of flies with supernumerary legs occurs. In a hot climate 

 there would be no evidence that the peculiarity was part 

 of the inheritance ; in a cold region it would be obvious. 

 This shows that the expression of the inheritance as 

 regards a particular character sometimes depends on 

 nurture. 



(2) While some developing organisms are strikingly 

 indifferent to changes in their environment, there are 

 others which respond sensitively, sometimes in a startling 

 way, to changes which do not seem very drastic. Mac- 

 Dougal's experiments l of injecting solutions of sugar and 

 of compounds of calcium, potassium, and zinc into the 

 developing ovaries of an Evening Primrose resulted in a 

 small percentage of notably atypical individuals, which 

 bred true to the third generation. The chemical reagents 

 introduced were not very different from those which might 

 occur naturally in the sap of the plant. Among the 

 changes induced there were not only losses and augmen- 

 tations of what was previously present, there were 

 distinct novelties which maintained their distinctness 

 when crossed with the parental strains. 



Loeb has recently shown that it is very easy to produce 

 a percentage of fish-embryos (Fundulus) with defective 

 eyes by adding a very minute quantity of potassium 

 cyanide to the water or by exposing the developing eggs 

 to low temperature. That is to say, relatively slight 

 environmental changes may so alter the constitution of the 

 developing embryo that a leap is taken in the direction 

 of blindness. Similarlv Stockard has shown for the 



/ 



same fish that the addition of a very minute quantity of 

 magnesium salt to the water induces in a large number 

 of embryos the development of a single Cyclopean eye in 

 place of the normal two eyes. 



Such cases are to be borne in mind in connection with 



1 In lecture on " The Direct Influence of Environment " in Fifty 

 Years of Darwinism (1909). 



