130 REGENERATION 



and pathological regeneration obviously depend on the same causes, 

 and often pass one into the other, so that no real line of demarca- 

 tion can be drawn between them. We nevertheless find that in 

 those animals in which the power of regeneration is extremely great 

 physiologically, it is very slight pathologically. This proves that a 

 slight power of pathological regeneration cannot possibly depend on 

 a general regenerative force present within the organism, but rather 

 that this power can be provided in those parts of the body which 

 require a continual, periodic regeneration ; in other words, the regen- 

 erative power of a part depends on adaptation." It is, I think, 

 erroneous to state "that in those animals in which the power of 

 regeneration is extremely great physiologically, it is very slight patho- 

 logically." All that we are justified in concluding from the evidence 

 is that in some cases in which physiological regeneration takes place, 

 as in the vertebrates, pathological (restorative) regeneration may not 

 be well developed ; but even in these forms restorative regeneration 

 is certainly present, and present especially in internal organs, as in 

 the salivary gland, in the liver, and in the eye, which are little exposed 

 to injury. How far physiological regeneration takes place in the 

 tissues of the lower animals we do not know at present, except in a 

 few cases, but far from supposing it to be absent, it may be as well 

 developed as in higher forms. Weismann's further conclusion, that 

 because in some animals physiological regeneration is very great and 

 restorative regeneration very slight, therefore the latter cannot "de- 

 pend on a general regenerative force within the organism," is, I 

 think, quite beside the mark. In this connection we should not fail 

 to notice a difference between these two regenerative processes that 

 several writers have also called attention to, viz. that the power of 

 cell-multiplication and the formation of new cells in each kind of 

 tissue does not carry with it the power of restorative or even of phy- 

 siological regeneration, in cases where several kinds of tissue make 

 up an organ. For instance, if the leg of the mammal is cut off, the 

 old cells may give rise to new ones, but the processes that would 

 bring about the formation of the new leg are not present, or, rather, 

 if present, cannot act. Thus, although the production of new cells 

 from each of the different parts of the leg of a mammal may take 

 place, yet the conditions are unfavorable to the subsequent formation 

 of a new leg out of the proliferated cells. We should not infer that 

 this power does not exist, but that under the conditions it cannot be 

 carried out. The assumption that physiological regeneration is the 

 forerunner of restorative regeneration, in the sense that historically 

 the. former preceded the latter and furnished the basis for the devel- 

 opment of the latter, cannot be shown, I think to be even probable. 

 This way of looking at the two processes puts them, I believe, in a 



