322 ORGANIC GROWTH 



as already remarked, by fibrils of protoplasm having the 

 ordinary appearance. 



We shall see that even in the lower multicellular animals 

 the conduction of nerve stimuli takes place without the 

 presence of actual nerves. I have only touched upon the 

 much less definitely understood relations in the ciliated In- 

 fusoria in order to indicate that there is a parallelism between 

 these and the important facts to be shortly discussed, and to 

 lead up to the latter. 



In the preceding I have principally referred to muscles and 

 nerves as evidence in support of my propositions, and shall 

 follow the same course hereafter, because these organs of so- 

 called animal life are specially suited to bring into view the 

 effect of the relations of the organism to the outer world. 



The essential distinction between the formation of organs 

 in the unicellular and multicellular forms is that in the 

 former the organs appear as parts of the cell, which is itself 

 the animal, while in the latter each organ is formed of 

 numerous cells. For instance, in the unicellular Vorticella a 

 part of the cell forms a muscle-fibre, in the multicellular 

 many cells form a muscle, which is physiologically equivalent 

 to that muscle-fibre. Morphologically, therefore, the two can 

 be no more compared than any other organs in the two 

 divisions of animals. But that they, like other organs in the 

 two groups, are physiologically equivalent, that in each 

 case, from a quite different starting-point, an appar- 

 atus acting in a similar way in relation to the outer 

 world has been produced, this alone warrants the conclu- 

 sion that it was this very relation to the outer world 

 which modified the protoplasm in both cases into an 

 equivalent though not homologous structure, that the in- 

 heritance of acquired characters is here exemplified. 



