v] NATURE OF PLAXT-ANIMALS 133 



changed being. Activity takes the place of lethargy 

 and growth, of degeneration. In a few days it be- 

 comes, instead of a microscopic, transparent object, a 

 visible, green organism. 



The immediate problem is, how to explain this 

 arbitrary behaviour of the uninfected organism. That 

 it is at once a pathetic tribute to the dependence 

 of C. roscoffensis on the infecting organism and a 

 justification of the title of this book, is evident. 

 Without the green cells, life to it is not worth living, 

 and it dies though surrounded by a plentiful micro- 

 flora of which in happier, infected circumstances it 

 avails itself without stint. 



Let us suppose that the tenor of normal develop- 

 ment of an organism is not smooth and even, but 

 abruptly intermittent ; that in the complex business 

 of growing up a business which involves many 

 simultaneous processes and many processes which are 

 necessarily consecutive the consummation of one 

 phase serves as the signal for the commencement of 

 the next. Then, if, for one cause or another, one 

 process does not complete itself, there will be no 

 signal for the beginning of the consequent process. 

 So, in respect of this series of processes, the organism 

 never grows up. It exhibits the phenomenon of 

 arrested development. The signal for full steam 

 ahead with the next growth-process may be produced 

 internally ; or it may be of external origin. The ivy, 



