318 ORGANIC GROWTH sec. 



The evidence for this fundamental law lies in the following 

 facts : (1) that numerous animals, although they possess no 

 organs structurally adapted thereto, regularly perform actions 

 which pre-suppose corresponding powers in the protoplasm, 

 or even imply that the evolution of such organs has already 

 begun ; (2) that various organs at first serve for many pur- 

 poses, and that division of labour, i.e. the increase in the 

 number of organs and their specific differentiation appears 

 only by degrees in the series of organisms ; (3) that the 

 larvae of even the most highly organised animals only 

 possess powers corresponding to those described under 1 and 

 2, — that, therefore, in this matter also the biogenetic law 

 completely holds good. 



The two first of the above propositions are sufficiently 

 proved by unicellular animals alone when their activities are 

 compared with those of the multicellular. 



The second proposition is further supported by the general 

 description of the evolution of organisation given above ; the 

 evidence required for the third can only be afforded by 

 multicellular animals. 



The striated layer in the Infusoria already mentioned is 

 also evidence of the truth of the first proposition. The 

 ciliated Infusoria which possess that layer exhibit the 

 commencement of the evolution of a muscular layer, which 

 commencement can only be ascribed to the constantly repeated 

 contraction in a given direction of that part of the body. 

 And in the stalk of the Vorticellidte, by the continuance of 

 the same activity, a muscular fibre has arisen, which behaves 

 physiologically in just the same way as the muscles of multi- 

 cellular animals. 



The actions of the ciliated Infusoria in relation to the 

 outer world are such that will must certainly be ascribed to 

 them. This is proved by the simple observation of the 

 character of their movements, apart from all else. Their 



