28 THE EVOLUTION THEORY 



the same ground, therefore, as that on which the whole determinant 

 theor}^ is based. The regeneration- determinants must be contained 

 as such in the germ-plasm, otherwise a twofold phyletic development 

 could not have occurred, as it actually has, in many parts. The tail 

 of the lizard is adapted for autotomy ; it breaks off when it is held 

 by the tip, and this depends on a special adaptation of the vertebrae, 

 which are very brittle in a definite plane from the seventh onwards. 

 This is thus a very effective adaptation to persecution by enemies. 

 The tail which has been seized remains with the pursuer, but the 

 lizard itself escapes, and the tail grows again. But this regeneration 

 does not take place in the same way as in the embryo; no new 

 vertebrae are formed, but only a ' cartilaginous-tube,' a new structure, 

 a substitute for the vertebral column ; the spinal cord with its nerves 

 is not regenerated either, and the arrangement of the scales is 

 somewhat different. 



This last point, in particular, indicates that the determinants of 

 the regeneration-rudiment may pursue an independent phylogenetic 

 path of their own, for this scale arrangement of the regenerated tail 

 is an atavistic one, that is, it corresponds to a more primitive mode of 

 scale arrangement in these Saurians. We know quite a number of 

 cases similar to this. It not infrequently happens that cut-off parts 

 regenerate, but that they do so not in the modern form, but in one 

 that is in all probability phyletically older. Thus the legs of 

 various Orthoptera, as of the cockroaches and grasshoppers, regenerate 

 readily, but with a tarsus composed of four joints instead of five ^, 

 and the long-fingered claws of a shrimp [Atyokla ijotiinirim) is 

 replaced by the older short -fingered type of claw, while in the 

 Axolotl an atavistic five -fingered hand grows instead of the amputated 

 four-fingered one. 



This last case shows that it is not merely a lesser power of 

 growth that accounts for the difference between the regenerated part 

 and the original, for here more is regenerated than was previously 

 present. There remains nothing for it but the assumption that the 

 regeneration-determinants have remained at a lower phyletic level, 

 while the determinants which direct embryogenesis have varied, 

 and either developed further or retrogressed. It is eas}^ to under- 

 stand that the regeneration -rudiment must vary phyletically much 



^ New investigations, specially directed to this point, by E. Godelmann, have 

 shown that *in the great majority of cases' the regenerated legs of a Phasmid 

 {Bacillus rossii) exhibit a four-jointed tarsus ; but the regeneration of five joints also 

 occurs, though only after autotomy, and only in seven out of fifty cases (Archiv fvr 

 Entivicklungstiiechanik, Ed. xii, Heft 2, July 1901). The regeneration-rudiment in this 

 species seems to be in process of advancing slowly to the five-jointed type. 



