152 THE GERM-PLASM 



viously to the division must be looked upon as a still later 

 modification of the process. 



This conclusion receives further support from the fact that, as 

 already shown, the capacity of regeneration is not by any means 

 an inherent quality of the organism : that is to say, it is not a 

 direct and inevitable result of a particular degree of organisa- 

 tion, but is due to an adaptation produced by natural selection, 

 and constitutes a special arrangement which may exist in 

 different degrees of perfection, or which may, again, be entirely 

 absent. If an earthworm is cut into two, the anterior portion 

 develops a new tail-end, but the posterior portion does not 

 give rise to a new head-end : the arrangement existing in 

 Lii7}ibriailus and Nais is therefore absent in this case. This 

 fact I should explain by assuming that in the last-named animals 

 the determinants required for the formation of the head-end 

 are supplied to the cells of the integument and alimentary canal 

 as accessory idioplasm, while in the earthworm these cells 

 only possess the determinants required for the formation of the 

 tail-end. 



It is very possible that the arrangement for the regeneration 

 of the tail-end may have taken place more easily than that 

 for the restoration of the head in the case of segmented 

 worms, owing to the fact that the last segment possessed the 

 power of giving rise to entire new segments. The growth of 

 the animal is effected by the formation of new segments at the 

 posterior end of the body, which would therefore be already 

 provided with the requisite accessory determinants, and it would 

 then only be necessary that these should be transferred to the 

 corresponding cells of the other body-segments. This might 

 have taken place in a relatively simple manner in the course of 

 philogeny, by a portion of the accessory determinants being 

 left in the cells of each new body-segment as it became formed. 

 The determinants of the head-end, on the other hand, can only 

 have been supplied to the respective cells as accessory idioplasm 

 before or during embryonic development ; we can therefore 

 understand why the capacity for forming a new head-end was 

 only acquired later, and that some worms are able to regenerate 

 the posterior, but not the anterior end by the body when it is cut 

 in half. 



We can therefore trace a series of stages of gradually increas- 

 ing complexity in the development of the process of regenera- 



