754 LIl^^E : OUTLINES OF GENERAL BIOLOGY 



that it is most marked in the simpler animals, and that it wanes in 

 the more differentiated. It is almost unlimited among sponges, 

 polyps, and certain kinds of worms; whereas it is almost unknown 

 among birds and mammals. The re-growth of new feathers after 

 moulting, or of new hairs after casting, can hardly be reckoned as 

 regeneration in the stricter sense, for they are the outcome of 

 papillse formed before the moulting or casting, much as next year's 

 foliage is developed from the buds formed in the axils of this year's 

 leaves. 



There are several common-sense reasons why the regeneration 

 of lost parts should be rare among highly differentiated animals. 

 {a) There is very little persistence of that indifferent or embryonic 

 tissue which is so abundant in simple forms, like Coelentera. (b) In 

 some structures of the more differentiated animals, ceU-division 

 has come to a standstill; which would tend to limit the possibility 

 of regeneration after an injury, unless some re- activation should 

 occur at the cut surface or below a protective cap. It may be 

 recalled that the number of nerve-ceUs in a Vertebrate brain does not 

 usually increase after birth; only in very rare cases is there any 

 trace of cell- division ; so this almost precludes regeneration after a 

 brain lesion, (c) Unused structures and capacities tend to disappear; 

 and as the higher animals are less liable to injury, in virtue of their 

 more effective nervous and locomotor organs — ^their wits and agility, 

 in fact — they have less need for the persistence of the regenerative 

 capacity, so important to polyps and starfish, {d) Again, if it be 

 granted, for the sake of argument in the meantime, that a particular 

 regenerative capacity is gradually evolved as an adaptation, we 

 cannot be surprised at its absence in animals where an injury that 

 implies some cutting or biting is likel}^ to be rapidly fatal. For if an 

 injury is rapidly fatal, a compensatory adaptation could not be readily 

 evolved. If a plump caterpillar is even punctured by a bird's bill, it 

 is likely to die rapidly, though it may escape being swallowed; it 

 is not surprising therefore that caterpillars should show almost no 

 regenerative capacit5^ Their characteristic protective adaptations, 

 e.g. resemblance to twigs, are directed towards the avoidance of 

 being even punctured. Starfishes and sea-urchins are not very far 

 apart, but whereas starfishes show remarkable regenerative capacity, 

 the only structures that sea-urchins can regenerate are their spines. 

 A reason for this is no doubt that the starfish's five-rayed body is 

 very liable to injury, while the sea-urchin's globe is not; in other 

 words, the only non-fatal injury to which a sea-urchin is liable is 

 the loss of spines. 



Yet the presence or absence of regenerative capacity is not wholly 

 a question of little or much differentiation. This may be shown by a 

 few contrasts. Nemertine worms are more highly differentiated than 

 Nematodes, yet regeneration is rife among the former, exceedingly 



