26 THE EVOLUTION THEOKY 



of what is useful or disadvantageous, and that it must take this course 

 accordino- to the given regeneration-mechanism and the stimulus 

 supplied in the special case. It cannot be supposed that these 

 supernumerary heads and tails are purposeful, but who would expect 

 an adaptive reaction from the animal in a case like this, since cuts of 

 the kind which we make artificially, and onust keep open artificially 

 if the deformities are to develop, hardly occur in nature, and, if they 

 did occur, would very quickly close up again 1 Adaptations can only 

 develop in response to conditions which occur and recur in a majority 

 of cases, and when they have a useful, that is, species-preserving 

 result. The adaptiveness of the organism is blind, it does not see the 

 individual case, it only takes into account the cases in the mass, and 

 acts as it must after the mechanism has once been evolved. The case 

 is the same as that of ' aberrant ' or mistaken instincts, whose origin 

 by means of selection is the more clearly proved, since we must 

 recognize such an instinct as a pure mechanism and not as the 

 outcome of purposeful forces. 



In the reofeneration of Planarians we must think of the reg-euera- 

 tion - idioplasm as containing the full complex of the collective 

 determinants of the three germinal layers, and possibly we must 

 add to this cells with the complete germ-plasm for giving rise to 

 the reproductive cells. But when the amputated tail of the newt 

 is regenerated, or its leg, or the arm of a starfish, or the bill of a bird, 

 we have no ground for assuming that the cells, from which regenera- 

 tion starts, contain the whole germ-plasm, since the determinants of 

 the replaceable parts suffice to explain the facts. We must even 

 dispute the possibility of the presence of the whole germ-plasm in 

 this case, because the faculty of regeneration of the relevant cells is 

 really no longer a general one, but is limited to the reproduction of 

 a particular part. This is seen in the fact that, in the starfish, whose 

 high regenerative capacity is well known, the central disk of the body 

 may indeed give rise to new arms ^ ; but an excised arm, to which no 

 part of the disk adheres, is in most starfishes unable to give rise to 

 the body. Thus the arm does not contain in its cells the determinants 

 of the disk, but the latter contains those of the arm. We are not 

 surprised that the amputated tail of the salamander does not reproduce 



^ I see now that there are contradictory statements in regai'd to this case. 

 Possibly these depend on the difTerent behaviour of different species, and this on the 

 varying frequency of mutilation. Starfishes which live on the shore between the 

 rocks, for instance on the movable stones of a bieakwater, are very frequently 

 mutilated ; in some places it is rare to find a specimen without traces of former 

 wounds. H. D. King counted among 1,914 specimens of Aslerkis vulgaris 206 in the act 

 of regenerating a i»art, that is, 10-76 per cent. In the case of the starfishes from deep 

 water this cause <if injury does not of course exist. 



