156 RE GENERA TION 



If we assume that the leg of some individual crayfishes or crabs, 

 for example, broke off, when injured, more easily at one place than at 

 another, and that regeneration took place as well, or even better, from 

 this region than from any other, and if we further assume that those 

 animals in which this happened would have had a better chance of 

 survival than their fellows, then it might seem to follow that in time 

 there would be more of this kind of animal that survived. But even 

 these assumptions are not enough, for we must also assume that this 

 particular variation was more likely to occur in the descendants of 

 those that had it best developed, and that amongst those forms that 

 survived, some had the same mechanism developed in a still higher 

 degree, and, the process of selection again taking place, a further 

 advance would be made in the direction of autotomy. This, I think, 

 is a fair, although brief, statement of the conventional argument as to 

 how the process of natural selection takes place. But let us look 

 further and see if the results could be really carried out in the way 

 imagined, shutting our eyes for the moment to the number of suppo- 

 sitions that it is necessary to make in order that the change may 

 occur. It will not be difficult, I believe, to show that even on these 

 assumptions the result could not be reached. In the first place, the 

 crabs that are not injured in each generation are left out of account, 

 and amongst these there will be some, it is true, that have the particu- 

 lar variation as well developed as the best amongst those that were 

 injured, and others that have the average condition, but there will be 

 still others that have the possibilities less highly developed, and the 

 two latter classes will be, on the hypothesis, more numerous than 

 those in the first class. The uninjured crabs will also have 

 an advantage, so far as breeding and resisting the attacks of their 

 enemies are concerned, as compared with those that have been injured, 

 and in consequence they, rather than the injured ones, will be more 

 likely to leave descendants. Even if some of those that have been 

 injured, and have thrown off the leg at the most advantageous 

 place, should interbreed with the uninjured crabs, still nothing, or 

 very little, can be gained, because, on Darwinian principles, inter- 

 crossing of this sort will soon bring back the extreme variations 

 to the average. 



The process of natural selection could at best only bring about 

 the result provided all crabs in each generation lose one or more of 

 their legs, and amongst these only the ones survive that break off the 

 leg at the most advantageous place ; but no such wholesale injury 

 takes place, as direct observation has shown. At any one time only 

 a small percentage, about ten per cent, have regenerating legs, and 

 as the time required completely to regenerate a leg, even in the sum- 

 mer, is quite long, this percentage must give an approximate idea of 



