130 EXPERIMENTAL EVOLUTION LECT. 



as they clearly show that while no difference at all 

 can be discovered in the external appearance of the 

 different types, considerable variation is present when 

 physiological properties are taken notice of. 



I have quoted this case at some length, because it is 

 one of the most satisfactory yet obtained ; but similar 

 instances are very numerous in bacteriology, where we 

 perceive that very considerable differences of a phy- 

 siological nature may exist although not perceptible 

 from the morphological standpoint. 1 



Zoology also provides us with other facts which are 

 of great interest. I refer to those which concern the 

 considerable physiological difference which obtains 

 between two closely related species, the brown and 

 the green frog (Rana esculenta and temporarid), when 

 subjected to identical experiment. 



In 1 88 1 Monnier 2 noticed that brucin and its 

 different compounds act differently on these two 

 species. In R. esculenta this alkaloid determines a 

 paralysis of the motor nerves, and at the same time 

 an increase in the excitability of the spinal cord. Of 



1 Cf. A. Chauveau : Sur les Proprietes vaccinates de Microbes ci-devant 

 fiathogenes transformes en Microbes d'apparence saprogene. Archives de 

 Medecine Exptrimentale, March, 1889, p. 161. Also, by the same 

 author : Recherches sur le Transformisme en Microbiologie pathogene. 

 Des Limites, des Conditions ct des Consequences de la Variabilitc du 

 Bacillus Anthracis. Ibid. November, 1889, p. 757. 



2 Archives des Sciences Physiques et Naturelles, Geneva, 1881. 



