134 EXPERIMENTAL EVOLUTION LECT. 



become equalised, increased reflex action and even 

 tetanic convulsions occurring in R. temporaria, and 

 distinct stiffness of the muscles becoming observable 

 in R. esculenta, although this stiffness never becomes 

 so great as in R. temporaria" 



The foregoing differences in the nervous system of 

 these two very closely related species are again exem- 

 plified in other experiments, for Lautenbach 1 has shown 

 that while the nerves of Rana temporaria are never 

 excited by heat lower than 49 centigrade, those of 

 R. esculenta are excited as soon as the temperature 

 attains or exceeds 20 centigrade ; and, on the other 

 hand, a friend of mine, M. C. Contejean, a dis- 

 tinguished young physiologist, informs me that, 

 according to his own experiments, considerable dif- 

 ferences are noticeable in individuals of the same 

 species which differ in colour. While frogs whose 

 skin contains numerous pigment granules withstand 

 for some time the effects of having part of their blood 

 replaced by salt solution, frogs whose skin is sparsely 

 coloured resist during a much shorter period. Again, 

 the same physiologist informs me that while Rana 

 esculenta and temporaria possess digestive glands in 

 the lining of their cesophagus, the toad has none. 

 Also, while the green and brown frog are provided 



1 The Physiological Action of Heat. Journal of Physiology, vol. ii. 



