THEORIES OF A U TO TOM Y 157 



the extent of exposure to injury. It is strange that those who assert 

 off-hand that, because autotomy is a useful process, therefore it must 

 have been acquired by natural selection, have not taken the pains to 

 work out how this could have come about. Had they done so, I can- 

 not but believe they would have seen how great the difficulties are 

 that stand in the way. 



A further difficulty is met when we find that each leg of the crab 

 has the same mechanism. If we reject as preposterous the idea that 

 natural selection has developed in each leg the same structure, then 

 we must suppose that a crab varies in the same direction in all its legs 

 at the same time ; and if this is true it is obvious that the principle of 

 variation must be a far more important factor in the result than the 

 picking out of the most extreme variations. The same laws that 

 determine that one individual varies in a useful direction farther than 

 do other individuals may, after all, account for the entire series of 

 changes. If it be replied that natural selection does not take into 

 account the causes of the differences of individual variation, this is 

 to admit that it avowedly leaves out of account the very principles 

 that may in themselves, and without the aid of any such supposed 

 process as natural selection, bring about the result. The Lamarckian 

 principle of use and disuse does not give an explanation of autotomy, 

 since the region of the breaking-joint is not the weakest region of the 

 leg, or the place at which the leg would be most likely to be injured. 



We cannot assume autotomy to be a fundamental character of liv- 

 ing things, since it occurs only under special conditions, and in special 

 regions of the body. While it might be possible to trace the autot- 

 omy of the legs of the Crustacea, myriapods and insects, to a common 

 ancestral form, yet this is extremely improbable, because the process 

 takes place in only a relatively few forms in each group. The au- 

 totomy of the wings of white ants that takes place along a preexisting 

 breaking-line must certainly have been independently acquired in this 

 group. The breaking off of the end of the foot in the snail helica- 

 rion is also a special acquirement within the group of mollusca. 



Bordage has suggested that the development of the breaking-joint 

 at the base of the leg of phasmids has been acquired in connection with 

 the process of moulting. He has observed that during this period the 

 leg cannot, in some cases, be successfully withdrawn through the 

 small basal region ; and hence, if it could not break off, the animal 

 would remain anchored to the old exoskeleton. It escapes at the 

 expense of losing its leg. The animal, having acquired the means of 

 breaking off its leg under these conditions, might also make use of the 

 same mechanism when the leg is held or injured, and thereby escape 

 its enemy. The fact that the crayfish has a breaking-joint only for 

 the large first pair of legs would seem to be in favor of this interpre- 



