SOME PROBLEMS OF REGENERATION. 205 



When the posterior end of an earthworm is cut off, there 

 appears at first at the posterior end of the anterior part a small 

 knob, out of which a few segments and a growing end develop. 

 The latter continues to extend backward and add new segments. 

 After a time the posterior growth comes to an end, i.e., when the 

 full complement of segments has been reached. If, however, 

 instead of the entire anterior end we take a small piece from 

 the middle of the worm, we find that even this piece only forms 

 as many new posterior segments as lay originally behind that 

 piece in the worm. In other words, the growing region ceases 

 to give rise to new segments after those that were lost have 

 been replaced, so that the entire number of segments in the 

 new worm may be far below the normal number. It seems to 

 me that we see here a factor at work that we cannot in the 

 least grasp. To call it correlation is of no avail unless we 

 can define what we mean by the term. The usual definition of 

 correlation is only a restatement of the facts in different words, 

 and not a causal explanation of the facts. 



It is not difficult to show how much we must include in our 

 definition of correlation if we use the word to cover all the 

 known facts. According to Blumenbach (1784), the eye of 

 Triton will regenerate if only a small part of the original eye be 

 ' left. If the anterior end of the earthworm be cut off very 

 obliquely, the pointed end will bend over the cut surface and 

 fuse with it, and slowly a new anterior end will develop between 

 the bent-over portion and the anterior cut surface. In both 

 these cases the relation of the new part to the old must be an 

 extraordinarily complex one, and yet a new organ develops like 

 the one lost. Again, pieces may be cut from the egg, and it 

 will still, in some cases, produce a whole animal of smaller 

 size a part developing a new whole much in the same way 

 that a part of a protozoon may form a new animal of smaller 

 size. Something more is included in these phenomena, I think, 

 than can be explained by simple physical interaction or by 

 chemical influences. The process that takes place suggests 

 that something like an intelligent process must be at work I 

 mean that what we call correlation of the parts seems here 

 to belong rather to the category of phenomena that we call 



