124 



THE GERM-PLASM 



to be accurately adjusted. Although we cannot easily judge as 

 to what is possible in nature, and are so often impressed by the 

 discouraging conviction that many vital processes are still incom- 

 prehensible to us, we may perhaps in this case feel justified in 

 itif erring the impossibility of such an occurrence from the fact 

 that it does not take place : that is, to infer that the regeneration 

 of a bird's wing is impossible on account of the complexity of 

 the mechanism required for it. because it does not actually 

 occur. 



We cannot, however, regard this as a formal proof of the fact 

 that regeneration does not take place in this case. This would 

 be inadmissible, if only because the first of the three factors which, 

 as we have assumed, produces the mechanism of regeneration — 

 that is, the probability of loss — is not present. In the state of 

 nature, at any rate, a bird's wing is seldom injured without loss 

 of life ensuing at the same time. For this reason alone, selec- 

 tive processes in connection with a regenerative mechanism 

 could not be introduced. I have not brought forward the 

 above example for the purpose of proving the case for this 

 instance in particular, but because it seemed to me to be spe- 

 cially fitted to show how^ extraordinarily the complexity of the 

 regenerative mechanism must increase along with the greater 

 complexity of the part. But this brings us back to the consid- 

 eration of the general power of regeneration possessed by the 

 lower ^ in contrast to the higher, animals. 



The supposition that this power exists, may, I believe, be con- 

 ceded in a certain sense : that is to say, in consequence of the 

 slighter complexity in structure of all the parts in one of the 

 lower groups of animals, any particular part may also become 

 capable of regeneration more easily than in the case of the higher 

 groups. We must, however, always presuppose that the two 

 other factors — the probability of injury, and the physiological 

 importance of the organ — are present in the required degree ; 

 so that in speaking of the greater power of regeneration pos- 

 sessed by animals of a lower type, we are only using another 

 expression for the third factor which takes part in the process, 

 viz., the complexity of the organ to be regenerated. 



The question, however, arises as to whether the capacity of 

 each part for regeneration results from special processes of adap- 

 tation, or whether regeneration occurs as the mere outcome — 

 which is to some extent unforeseen — of the physical nature of 



