THE DEVELOPMENT OF ORGANISMS 755 



rare among the latter. Regeneration is common among amphibians, 

 especially among newts, yet it is very rare among fishes. A newt can 

 regenerate most of its leg; a fish cannot even replace a lost scale. 

 Or again, a crab's leg is a structure far more complex than a bird's 

 claw, yet the replacement of a crab's leg is a common occurrence, 

 whereas the replacement of a torn-off claw in birds is almost unknown. 

 The annual replacement of the claws of grouse and ptarmigan 

 (which is all but peculiar to these) is a normal renewal, not a reaction 

 to casual wounding. To take one other instance, earthworms and 

 leeches are not on any very dissimilar level of differentiation, yet 

 regeneration is common and extensive in the former, but limited 

 and very rare in the latter. 



If we regard the regenerative capacity as due to the diffuse or 

 localised persistence of some more or less embryonic tissue, or of 

 rejuvenescent cells whose formative factors are readily activated by 

 the stimulus of violence, the puzzle remains — why this capacity 

 should persist in certain types and not in others. According to Lessona's 

 suggestion (1868), afterwards elaborated by Weismann, the regenera- 

 tive capacity tends to persist in those animals and in those parts 

 of animals which are, in the natural conditions of their life, peculiarly 

 liable to non-fatal and frequently recurrent injury. Tliis suggestion, 

 ecological, not physiological, is known as Lessona's Law. As Weis- 

 mann stated it: "The power of regeneration possessed by an animal 

 or by a part of an animal is regulated by adaptation to the frequency 

 of loss, and to the extent of damage caused by the loss." 



In support of this view that the distribution and persistence of 

 the regenerative capacity is adaptive, and to be accounted for as the 

 outcome of Natural Selection, a nimiber of concrete examples may 

 be adduced. The autotomy and regeneration of the lizard's tail are 

 very general, in fact almost characteristic of the order. Yet neither 

 occurs in Chamaeleons; a fact which may be reasonably correlated 

 with this highly specialised lizard's habit of coiling its tail round a 

 branch. A few other exceptions have been noted; yet more or less 

 similarly explained; thus Werner points out that the regeneration 

 is absent, or very incomplete, in those lizards, like the African 

 Zonurus, which use their tails in defence. A tail used in striking 

 would be less likely to be seized than the ordinary wagging and 

 partially locomotor tail of ordinary long-tailed lizards. 



Further survey of cases of regeneration shows its frequent occur- 

 rence among long-limbed or long-bodied animals, such as starfishes, 

 brittle-stars, higher crustaceans, sea-spiders, ribbon-worms, and 

 limbless lizards. As Lessona said, these are all peculiarly liable to 

 the frequent recurrence of non-fatal injuries. Some difficult cases 

 remain, no doubt, yet some of these exceptions not only test but 

 eventually corroborate Lessona's rule. Spiders are notably long- 

 limbed, and captured specimens sometimes show autotomy followed 



