556 THE WONDER OF LIFE 



what we may call a process of continuous growth. To 

 meet the exigencies of normal life, the daily wear and tear, 

 there is a continuation of local growth, which has certainly 

 its bearing on the regeneration of accidentally lost parts. 

 One of the noteworthy limitations of this tissue-regenera- 

 tion concerns the nervous system in higher animals, for 

 the number of nerve- cells does not appear to admit of any 

 increase after birth. 



Thirdly, it seems useful to remember that the process 

 of asexual reproduction which many organisms exhibit 

 is a utilization of the capacity of re-growing the whole 

 from a part. Among Stinging Animals, Annelids, Polyzoa, 

 and Tunicates there is often a normal giving off of a portion 

 of the parent, which develops into a new individual. It 

 may be a bud or an area of the body or a fragment it is 

 a part which is capable of re-growing the whole. The pro- 

 cess is certainly to be brought into line with the regenera- 

 tion of a lost part and with the regeneration of an entire 

 individual from an artificially excised part. 



It is instructive to consider cases where the power of 

 asexual reproduction, which is normally a mere alternative, 

 may become the main or exclusive means of continuing the 

 species. This is well illustrated by the case of an Alpine 

 Planarian, studied by Voigt. It is a relict of glacial con- 

 ditions, a northern form, abounding in the hill streams 

 around Bonn. Observation and experiment show that 

 warmth hinders its sexual reproduction ; out of 4,000 speci- 

 mens not one was sexual ; it is keeping its foothold in 

 existence solely in virtue of its power of multiplying by 

 division. In other words, a power which is, to begin with, 

 only a subsidiary alternative, may in special circumstances 

 become of essential importance. This should be borne 



