94 



THE GERM-PLASM 



efficient protection in many of the lower animals — more espe- 

 cially in polypes and worms — than would the possession of 

 shells, stings, poison-organs, and all other kinds of weapons, or 

 even protective coloration. For although all these arrange- 

 ments certainly serve as a protection from many enemies, and 

 from various dangers, they are not always effective, and there- 

 fore the capability of restoring losses of substance would cer- 

 tainly be extremely valuable in any case. This fact must not be 

 forgotten in any inquiry with regard to the question of regenera- 

 tion. If we consider how highly important regeneration is from 

 a physiological point of view, its wide and even general distribu- 

 tion in the animal kingdom need not surprise us, and we shall 

 be able to understand why it has been introduced even into the 

 course of normal life : for the functions of certain organs depend 

 on the fact that their parts continually undergo destruction, and 

 are then correspondingly renewed. In this case it is the process 

 of life itself, and not an external enemy, that destroys the life 

 of a cell. I refer, of course, to the process of physiological 

 regeneratioti . 



Our knowledge of histology is not yet sufficient for us to be 

 able to determine what tissue-cells in the higher animals become 

 worn out by use during life, and have therefore to be continually 

 replaced ; but it has been proved in many cases that the wear- 

 ing away of the cells goes on incessantly, and that life could not 

 last if these cells were not constantly replaced. Such a constant 

 loss and renewal of the cells occurs in the cases of the epidermis of 

 the higher \^ertebrates, the human finger-nails, blood-corpuscles, 

 hairs and feathers, claws and hoofs, the epithelial lining of the 

 respiratory and other passages, and even in the antlers of stags. 

 In all these cases a continual or periodic wasting away or shed- 

 ding of groups of cells occurs normally, and a corresponding 

 replacement of these cells is one of the normal functions of the 

 body, and is therefore provided for. 



It is not difficult to explain the simplest of these cases of 

 physiological regeneration theoretically. If a tissue such as the 

 human epidermis, for instance, consists of one kind of cell only, 

 it is only necessary, in order that regeneration may take place, 

 that all these cells should not be thrown off simultaneously, and 

 that the tissue should be composed of cells of various ages, the 

 vounjrest of which, under certain influences of nutrition and 

 pressure, always retain the power of reproduction, and so form 



