REGENERATION AND LIABILITY TO INJURY 93 



ing case in favor of this view is that the terrestrial salamander, accord- 

 ing to Lessona, cannot reproduce lost parts, whilst another species 

 of the same genus, the aquatic salamander, has extraordinary powers 

 of regrowth, as we have just seen ; and this animal is eminently liable to 

 have its limbs, tail, eyes, and jaws bitten off by other tritons." 



Lang, referring to the brittleness of the tails of lizards, points out 

 that this is a very useful character, since the bird of prey that has 

 struck at a lizard gets hold of only the last part of the animal to dis- 

 appear under cover ; the lizard escapes by breaking off its tail. The 

 brittleness of the tail is, therefore, an adaptive character that has 

 become fixed by long inheritance. 



To this example may be added that of certain land snails in the 

 Philippine Islands. The individuals of the genus helicarion live 'on 

 trees in damp forests, often in great droves. They are very active, 

 and creep with unusual swiftness over the stems and leaves of the 

 trees. Semper has recorded that all the species observed by him 

 have the remarkable power of breaking off the tail (foot) close behind 

 the shell, if the tail is roughly grasped. A convulsive movement is 

 made until the tail comes off, and the snail drops to the ground, where 

 it is concealed by the leaves. Semper adds that in this way the snails 

 often escaped from him and from his collectors, leaving nothing behind 

 but their tails. The tail is said to be the most obvious part of the 

 animal, and it is assumed that this is, therefore, the part that a rep- 

 tile or bird would first attack. 1 Lang states that in this case external 

 influences have produced an extraordinarily well-developed sensitive- 

 ness in the animal, so that it reacts to the external stimulus by volun- 

 tarily throwing off the tail. It would be, of course, of small advantage 

 to be able to throw off the tail unless the power of regenerating the 

 lost organ existed, or was acquired at the same time as the extreme 

 sensitiveness that brings about the reaction. Lang does not state, 

 however, explicitly that he believes the regenerative power to have 

 arisen through the exposure of the tail of the lizard and the tail of 

 the snail to injury, although he thinks that the mechanism by means 

 of which these parts are thrown off has been acquired in this way. 

 Several other writers have, however, used these same cases to illus- 

 trate the supposed principle of liability to injury and power of 

 regeneration. 



Weismann in his book on The Germ Plasni has adopted the 

 principle of a connection between regeneration and liability to in- 

 jury and has carried it much farther than other writers. We can, 

 therefore, most profitably make a careful examination of Weismann's 



1 Whether, having once failed in this way to obtain the snail, the bird or lizard would 

 not learn to make a frontal attack is not stated. Or shall we assume that the tail is all that 

 is wanted? 



