I08 THE GERM-PLASM 



how slightly the diversity of external influences affects the 

 development of an organism. How wonderfully accurately the 

 course of ontogeny must be prescribed, if it can be kept to so 

 closelv. through thousands of generations of cells, that •• identi- 

 cal ' twins result ! We may compare the process of development 

 of such twins with the course taken by two ships, which, start- 

 ing from the same place, proceed along the same devious route 

 whiqh has been carefully mapped out beforehand in all its 

 thousands of definite changes in direction, until each finally 

 reaches the same distant shore independently, within a mile of 

 the other. 



A careful consideration of such a case as this leaves no doubt 

 that a very exact and definite course is mapped out for the egg- 

 cell by its idioplasm, which, again, directs the special course to 

 be taken by each of the innumerable generations of cells, in the 

 direction of which course external influences can only play a very 

 subordinate part. If this consideration be borne in mind, it will 

 be less likely that the objection may be made that a much too 

 complicated stmcture has been attributed to the idioplasm. Its 

 structure must be far more complex than we can possibly imagine ; 

 and in this respect, its construction, as we have represented it 

 theoretically, must certainly be far simpler than is the case in 

 reality. For the same reason, it is less probable that similar 

 objections may be made to the theory of regeneration as here 

 stated. Complicated phenomena cannot possibly depend on a 

 simple mechanism. The machines in a cotton factory cannot 

 be constructed of a few simple levers, nor can a phonograph be 

 manufactured from two lucifer matches. 



That form of regeneration which has been considered above 

 may be described as palingenetic^ for it pursues the course taken 

 by the primary or embryonic development ; but as soon as it 

 leaves this course and takes a shorter one, it may be distin- 

 guished as coenogenetic. 



Coenogenetic variations of the primary process of development 

 probably always occur in cases of regeneration of complex struct- 

 ures ; and even the reconstruction of the extremities, which we 

 have chosen above as an example, will hardly take place in 

 exactly the same way as occurs in the primary development of 

 these parts, although it may resemble the latter in its principal 

 phases. 



Even if mere abbreviation of the development of a part can 



