. 28 PROBLEMS OF EVOLUTION 



character. The only questions that remain, then, are : how far 

 is the germ-plasm localised ? How far is it present, in a dormant 

 state, in the body cells generally? In other words, how far 

 has specialisation gone on ? To what extent have the cells of 

 the soma lost their power of building up a complete individual ? 

 These after all are questions of detail. They do not touch the 

 question of absolute continuity. 



Reproduc- Some of the chief difficulties that Hertwig mentions will show 

 parts tn * 8 ' Nais, one of the loosely defined group of worms, has 

 an extraordinary power of regenerating lost parts. If it be 

 cut in half, the anterior end reproduces the posterior : the 

 posterior end reproduces the anterior. It is commonly supposed 

 that the same phenomenon occurs with earth-worms, and that 

 you can make one into two. This, however, is not the case. 

 The front end will reproduce the hinder end, but the more 

 wonderful phenomenon of the reproduction of the chief body 

 organs from the posterior end does not show itself. However, 

 in Nais it is an undoubted fact. There are many other less 

 striking instances of regeneration. If a crayfish loses a claw, a 

 new one is soon to be seen taking exactly the form of the lost 

 limb. When a lizard loses its tail it grows again. In Tritons 

 and Salamanders the whole limb grows, if it is amputated. In 

 the former even a jaw is replaced. This is an astonishing fact, 

 but it is very different from what happens in Nais. Low down 

 in the animal kingdom we have frequent examples of regenera- 

 tion. But when we ascend to the higher forms of animal life, 

 such phenomena almost disappear. The following instance is an 

 almost solitary one from a class so high as birds. 1 A stork had 

 the upper part of its beak accidentally broken off, whereupon 

 the lower portion was sawn off at the same point : both parts in 

 time grew again. Certainly when we descend low in the scale, 

 we find such cases more frequent, but we cannot say that the 

 power of regeneration is in inverse proportion to the degree of 

 organisation, for no such power seems to exist in fish which are 

 lower in the scale than Tritons and Salamanders. 



Regeneration in the ordinary course of life, not in consequence 



1 See Weismann : Germ Pimm, p. 125. 



