REGENERATION 29 



more slowly than the parts which evolved in the ordinary way 

 and much more slowly than the determinants of these parts, for 

 natural selection means a selection of the fittest, and the speed with 

 wdiich the establishment of a variation is attained depends, ceteris 

 'paribus^ on the number of individuals that are exposed to selection 

 Avith respect to the varying part. If in a species of a million 

 living at the same time nine-tenths perish by accident, there Avill 

 remain only 100,000 from which to select the 1,000 which we will 

 assume constitute the normal number of the species. The more of 

 these T 00,000 w^hich possess the useful variation the higher wdll be 

 the percentage of the normally surviving 1,000 possessing it, and 

 the more rapidly will the useful variation increase. But when it 

 is a question of the variation of the regeneration-primordium, the 

 selection will take place not among all the ioo,coo individuals which 

 chance has spared, but only among those of them which have lost 

 a limb by accident, and thus are in a position to regenerate it more 

 or less completely. If we assume that this takes place in 10 per cent, 

 of cases, then selection for the improvement of the regeneration- 

 apparatus will only take place among i,oco individuals, and thus the 

 process of modification of the regeneration-primordium must go on 

 very much more slowly than that of the limb itself. 



I do not see how the opponents of the germ-plasm theory can 

 explain these facts at all, for the appeal to external influences is 

 here entirely futile, and that to internal liberating stimuli does not 

 suffice, since these must be different after a part has been cut off" from 

 what the}^ were when the limb developed normally, and also different 

 from those which prevailed at the normal origin of the limb in 

 ancestral forms. The four -jointed tarsus of the ancestors of our 

 cockroaches did not arise as a result of amputation. We cannot 

 therefore avoid referring the processes of regeneration to particular 

 ' regeneration-determinants/ which are contained in the germ-plasm 

 and are handed on in ontogeny with the other determinants from 

 cell -division to cell-division, till ultimately they reach the cells which 

 are to respond, or may have to respond, to the stimulus of injury by 

 some expression of their regenerative capacity. As these deter- 

 minants, as has been shown, can often only be very slightly subject 

 to the influence of selection processes, they will, in many respects, lag 

 behind in the phyletic development, and will tend to belong to an 

 ancestral type of the relevant part. They will often remain for a long 

 time at this ancestral level, and they will always adapt themselves to 

 new requirements more slowly than the parts which arise in tlie 

 normal way, and the determinants representing these in the germ. 



