USE AND DISUSE 31 



especially the very young embryo, it persists in a greater 

 degree. Thus, if one of the two cells into which the frog's 

 ovum first divides be destroyed, the remaining cell may 

 reproduce the whole tadpole, though of a half-size. Later in 

 development the tadpole is able to reproduce an amputated 

 tail, a power of regeneration much greater than any possessed 

 by the adult frog. Low in the scale of life, in the adult, as 

 we have just seen, a fragment of sponge, for instance, is able 

 to reproduce the whole ; higher in the scale, a star-fish can 

 reproduce a ray, a lobster a claw, and so forth ; but none of 

 these parts can reproduce the whole ; that is done solely by 

 germ-cells. Higher yet, as among birds and mammals, the 

 power of reproducing lost parts is comparatively very trifling. 

 Important and complex parts cannot be restored. Wounds 

 and mutilations are healed ; but, if important, very imper- 

 fectly, for mere scar tissues replace the tissues that were 

 lost. 1 



53. It is clear, then, the reproduction of lost parts, whether 

 it be on a very great and perfect scale, as when a fragment 

 reproduces a whole, as in a sponge, or whether it be on a very 

 small or imperfect scale, as when a wound is healed in one of 

 the higher animals, is a process of the same order. It is, in 

 fact, a process of the same order as the reproduction of an 

 entire organism from a germ-cell Now we speak of a scar, 

 in a man for example, as an acquired character ; but who would 

 dream of speaking of all that is reproduced by a fragment of 

 a sponge or a begonia leaf, or indeed by a fertilized germ-cell, 

 as a character acquired by the fragment or the germ-cell? 

 Moreover, when one of the higher animals is mutilated, as 

 when a dog loses his tail, we lump together both the mutila- 



1 The above, however, expresses only a part of the truth, and probably 

 only a small part of it. It is true that the power of regeneration is very 

 generally lost to the highly specialized tissues of the higher animals, but 

 apparently it is not lost solely (if to any extent) on account of the high 

 exaltation of one special function in each cell. This exaltation of one 

 function does not appear necessarily to entail the loss of other functions. 

 On the contrary, the principal, if not the sole, reason of the loss appears 

 to be that the retention of the power of regeneration is no longer very 

 useful. For obvious reasons the fragment of an antelope could not 

 maintain existence while it was regenerating the whole animal ; even the 

 loss of a limb entails inevitable destruction of the whole. Even among 

 allied species of animals the power of regeneration varies in proportion 

 to its usefulness. Thus terrestrial salamanders have little power of 

 regeneration, " whilst another species of the same genus, the aquatic 

 salamander, has extraordinary powers of re-growth . . . this animal is 

 eminently liable to have its limbs, tail, eyes and jaws bitten off by 

 other Tritons." (Darwin, Plants and Animals, vol. ii, pp. 358-9. 

 Compare Weismann, The Germ-plasm, p. 114.) 



