vii EVOLUTION OF PROTOZOA 317 



layer becoming striated or fibrillar. The fibrils are always 

 arranged in the direction which enables them most easily to 

 produce the usual contraction of the body, viz. perpendicu- 

 larly to the plane of this contraction. In the stalk of the 

 bell animalcules (Vorticellidee) the fibre to which the con- 

 traction of the stalk is due, according to the researches of 

 Kiihne, has exactly the same physiological properties as the 

 muscular substance of multicellular animals. 1 



Thus these ciliated Infusoria are cells with a very delicate 

 organisation, which must have been gradually developed in 

 the course of ancestral evolution, and this certainly cannot 

 have taken place as a result of the variation of their germ- 

 cells, for the simple reason that they do not possess any. It 

 must have taken place in consequence of acquirements due 

 to use, and in consequence of the inheritance of such acquire- 

 ments. 



It is most clearly shown in the appearance of a specially 

 differentiated layer of contractile substance beneath the 

 surface of the body, and in the appearance in the stalk of the 

 Vorticellidae of muscle substance equivalent to that of the 

 higher animals, that activity or function calls forth organic 

 or physiological differentiation. 



This important proposition, which I might call the ele- 

 mentary fundamental law of all biological science, implies 

 the most complete opposition to the denial of the inheritance 

 of acquired characters, affirms the opposite. 



Protoplasm has the property of being altered and trans- 

 formed ly the action of external stimuli. 



Among external stimuli are to be understood both those 

 acting directly and those which act indirectly by affecting 

 the functions of the organism. 



1 " The stalk of the Vorticellidae behaves exactly like frog's muscle : even when 

 isolated from the rest of the animal it can be made to contract, and even be 

 tetanised, by the stimulus of variations of an electric current," etc. Cf. W. Kiihne, 

 Myologische Uniersuchungen, Leipzig, 1860, p. 216 or 213. 



