PROCESS OF REPRODUCTION IN ORGANISMS. 35 



we shall be able to effect the establishment of such equilibria 

 more readily. 



Moreover, the apparent independence of the germ cells is 

 readily accounted for in another way, viz., by the fact that they 

 undergo dedifferentiation and rejuvenescence after isolation 

 and fertilization. In this process only the most stable char- 

 acteristics of the specific protoplasm acquired during their 

 history as a part of the organism remain and all else is eliminated. 

 Two different parts of the planarian body, for example, with 

 different degrees and kinds of differentiation, both produce, 

 when isolated under the usual conditions, individuals with the 

 same general specific characters, though some minor differences 

 connected with the different origin of the pieces may persist. 

 There are, however, certain limits to the process of regulatory 

 reproduction in Planaria and other forms and an investigation 

 of these limits and the conditions which determine them promises 

 results of much interest for the problem of inheritance. On the 

 other hand, it is possible to demonstrate experimentally that 

 changes in the hereditary capacity of parts of the planarian 

 body can be induced by changing their position with respect 

 to other parts. If, for example, we cut off the anterior half 

 of the planarian body and allow a head to form at the anterior 

 end of the posterior half, we find that the capacity of the region 

 just posterior to the new head for developing a head when iso- 

 lated is very greatly increased by its changed position in the body. 



The further elaborations of the germ plasm hypothesis, the 

 hypotheses of determinants and of unit characters as repre- 

 sented by discrete independent elements, are wholly unneces- 

 sary. Far from assisting us in analyzing and interpreting the 

 phenomena of inheritance, they only complicate the problem, 

 for if they exist they are the most remarkable entities in the 

 world. Their assumed existence makes real progress in the 

 solution of the problems involved almost impossible, for we 

 can juggle with them as the facts seem to demand and there 

 is none to say us nay, since they are beyond the limits of scien- 

 tific investigation. The apparent independent variation of 

 characters, the Mendelian phenomena, the association or 

 coupling of characters, sex-limitecl inheritance and in fad all 



