RELATION OF THE NEMERTEA TO THE VERTEBRATA. 611 



power of reproduction of lost parts comes, without doubt, under 

 the general laws of formation and growth. We find it even 

 in the lowest Protozoa. If the material which heredity has 

 accumulated, either in such a unicellular being or in the egg 

 of a Metazoon, and out of which the elements of the different 

 organ systems will gradually develop, is hereditarily so disposed 

 that a compensation for the loss of important parts is facili- 

 tated, this will, of course, constitute an advantage. Such a 

 compensation may, e.g. be obtained where the generative 

 products are developed in very many separate centra, and not 

 in one closed sac. Injury to the latter will, ceteris paribus, 

 be more fatal than an equivalent injury destroying one or more 

 of the former. The same holds good for diflFused instead of 

 concentrated nervous centra, for the case of liver saccules to 

 the intestine, instead of one compact liver, for numerous aper- 

 tures and deferent ducts to the nephridial system instead of 

 one, &c. And all this is still more evident when we have 

 before us a long, bilaterally symmetrical animal, which is easily 

 snapped in two. In this case it must be of pre-eminent impor- 

 tance, that the remaining halves, which may in their turn be 

 severed by the same cause into smaller parts, should possess 

 sufficient power of reproduction to repair the damage. Now, it 

 cannot be doubted that an equal distribution of the important 

 components of the organism (nervous centra, generative organs, 

 nephridia, intestinal appendages, &c.) throughout the whole 

 length of the animal meets this requirement. Any severed 

 portion will then be provided with these more important parts, 

 and will be more or less adapted for a separate and individual 

 existence. 



The formation of a new mouth and of new brain-lobes in a 

 fragment of this description remains, of course, quite as won- 

 derful and inexplicable as before, but still we cannot fail to see 

 that such an arrangement as here indicated must somehow be 

 beneficial to the species, and that we need not stop short with 

 Bateson,^ when he says that " the repetition of various struc- 



1 Bateson, " The Ancestry of the Chordata," ' Quart. Journ. Micr. Sci.,' 

 vol. xxvi, pp. 545, 54G, 1886. 



