160 SCIENCE AND PHILOSOPHY OF THE ORGANISM 



Imagine that you have a piece of paper before you and 

 wish to sketch a landscape. After drawing for some time 

 you notice that you have miscalculated the scale with 

 regard to the size of the paper, and that it will not be 

 possible to bring upon the paper the whole of the landscape 

 you want. What then can you do ? You either may 

 finish what you have begun to draw, and may afterwards 

 carefully join a new piece of paper to the original one and 

 use that for the rest of the drawing ; or you may rub out 

 all you have drawn and begin drawing to a new scale ; or 

 lastly, instead of continuing as you began, or erasing 

 altogether, you may compromise as best you can by draw- 

 ing here, and erasing there, and so you may complete the 

 sketch by changing a little, according to your fancy, the 

 proportions as they exist in nature. 



This is precisely analogous to the behaviour of our 

 Tulularia. Tubularw also may behave in three different 

 ways, if, as I described to you, the terminal one of its two 

 newly arisen rings of tentacle primordia is removed again. 

 It may complete what is left, say the basal tentacle ring, 

 then put forth from the horny skeleton (the " perisarc ") the 

 new head as far as it is ready, and finally complete this 

 head by a regular process of budding regeneration. But it 

 also may behave differently. It may " erase " by a process 

 of retro-differentiation all that has been left of what had 

 already been formed, and then may form de novo the 

 totality of the primordia of a new head. Or, lastly, it 

 may remove a part of the middle of the one ring of tentacle 

 rudiments which was left, and may use this one ring for 

 the formation of two, which, of course, will not be quite in 

 the normal relations of place with regard to each other and 



