EVOLUTION IN NATURE 157 



lows: The difference in length of tail between 

 normal stock and young reared at normal tempera- 

 tures from treated stock was thirteen per cent of 

 the difference between the treated parents and the 

 normal stock; in foot length the difference was 

 twenty-six per cent; in ear, sixty-three per cent. 

 Simultaneously with the publication of these 

 figures Sumner suggested: *'It might be argued 

 that the very plasticity of a part, which makes it 

 so responsive to outside influences, might render 

 it correspondingly ill adapted to retaining such 

 impressions permanently. Such speculations are 

 decidedly premature, however." ^^ Regardless of 

 its standing as a conclusion from experimental 

 evidence, such an interpretation is logical; the 

 results are of the kind which might well be pre- 

 dicted of the responses of plastic structures to 

 environmental influences. A valuable feature of 

 these experiments is the fact that the mouse, 

 a mammal, maintains a constant temperature, 

 hence any suggestion of parallel induction must 

 be abandoned. 



Agar worked with Simoce'phalus vetulus, a phyl- 

 lopod crustacean, recording the ratio of total 

 length of newborn individuals to the width between 

 the ventral edges of the valves of the carapace.^* 

 When fed certain foods, the valves rolled back, 



" Am. Nat, Vol. XLIV, pp. 5-18, 1910. 



^^Ptoc. Roy. Soc. Lond., Vol. 86B, pp. 115-116, 1913. 



