THE CHARACTERISTICS OF ORGANISMS 23 



such replacement of cells there is an easy transition to the re-growth 

 of lost parts. The starlish re-grows its lost arm, the crab its claw, 

 the snail its horn, the earthworm its head. From cells below the 

 plane of separation there is in each case a regulated growth, which 

 replaces what has been lost. We have already mentioned a very 

 striking instance, in which re-growth is normal, and in organic and 

 seasonal rhythm independent of any violence from without — 

 namely, the re-growth which gives the stag new antlers to replace 

 those of the previous year. This capacity for regenerating lost parts 

 is mostly restricted among mammals to superficial structures like 

 hairs; as in the birds to renewing feathers. The reason for this 

 restriction is primarily that highly differentiated cells lose their 

 power of dividing: thus nerve cells are not regenerated in back- 

 boned animals, though when a nerve is cut the fibres, in continuity 

 with the central system, may repair the breakage by sending out 

 delicate processes which feel their way with amoeboid tips, towards 

 rejoining the separated portion. Another reason is to be found in 

 the adaptiveness of the regenerative process. As we shall illustrate 

 more fully in the chapter on reproduction, regenerative processes 

 tend to occur in those animals, and in those parts, which, in the 

 natural conditions of their life, are peculiarly liable to recurrent 

 non-fatal injury. In other words, the needful renewal of embryonic 

 tissue is rarely seen, unless there be some recurrent need for it. 

 Most lizards can re-grow their long tail if that has been snapped off 

 by a bird or surrendered in fear or in battle, but the chameleon, 

 which keeps its tail coiled round the branch, has not unnaturally 

 lost this power. Long-limbed animals like crabs, and starfishes 

 with their lank arms, have great regenerative capacity, in striking 

 contrast to the compact and swiftly moving fishes, which cannot 

 even replace a lost scale! The recurrence of non-fatal injuries is not 

 common among the higher animals, so their power of regenerating 

 important parts has waned. Enough of this, however; our present 

 point is that the regeneration of lost parts illustrates a renewal of 

 that regulated growth of complicated structure which is charac- 

 teristic of embryonic development. Out of apparently simple cells 

 at the stump of a snail's horn, the whole can be re-grown, including 

 the eye at the tip; and this may occur not once only, but forty 

 times. From the broken portion of a Begonia leaf there buds a 

 complete plant — to root and shoot and flower. From such recon- 

 structions there is but a step to the asexual multiplication of many 

 plants and animals — whether by the bulbils of the lily, the budding 

 of the hydra in the pond, or the halving of the planarian worm. 

 When the tail-half of the dividing planarian worm proceeds to 

 differentiate a new head, with brain-ganglia, eyes, and mouth 

 complete, there is an obvious development — the formation of new 

 and complex structures out of the undifferentiated and apparently 



