RECIEXERATION 121 



that the regeneration of a limb is a greater feat than the renewal 

 of feathers or antlers ; and the fact has been long recognised, 

 that the more complex organs are regetierafed less easily than 

 those which have a simpler structure. A series of carefully 

 performed experiments, made with the view of testing this 

 somewhat vague statement, would be of great value theoreti- 

 cally. We may predict that in one sense it would be confirmed, 

 and that we should find that under similar conditions the 

 simpler organs are on the whole regenerated much more easily 

 than the more complex ones in any particular species. Even 

 in the human race, many simple tissues — such as the connective 

 substances, epithelia and nerves — can be repaired, and it is 

 only the cells of the glands and ganglia, which are the most 

 highly differentiated histologically, which are not replaced at 

 all. or at most only to a very slight extent. We can see from a 

 theoretical point of view that a far less complex apparatus is 

 required in these cases than in those which concern a regenera- 

 tion of entire parts of the body, such as the tail or limbs ; for it 

 is only necessary that the respective tissues should contain cells 

 which are capable of multiplying, in response to the stimulus 

 produced by the loss of substance in their immediate neighbour- 

 hood, and which continue to do so until the loss is made good. 

 When, however, several kinds of cells take part in the restora- 

 tion, and a strict regulation as to their arrangement in groups, 

 their direction of growth, and rate of reproduction is required, 

 it becomes necessary for the individual cells from which the 

 restoration takes place to be accurately provided with supple- 

 mentary determinants of various kinds ; and it is clear that this 

 will gradually become more difficult and complex, the greater 

 the complexity of the part to be regenerated, and the more 

 accurately all the details of its stmcture have to be preserved. 



If, however, we review the facts known to us concerning 

 regeneration in animals of various degrees of organisation, we 

 meet with such marked differences even as regards the regenera- 

 tive power of homologous parts, that we cannot help receiving 

 the impression, which has affected all writers on this subject, 

 that in general the regenerative power is greater in less highly 

 organised animals than in those of a more complex structure. 

 The question thus arises as to how this view is to be interpreted 

 and presented in a scientific form. 



Even in Vertebrates, certain facts seem to indicate that the 



