758 LIFE : OUTLINES OF GENERAL BIOLOGY 



activating stimuli are, and why they are present in certaia types 

 and parts of the body and not in others. 



It seems reasonable and not an evasion of the question to say 

 that some types have more persistent embryonic capacity than 

 others. The ever-youthful meristem tissue in plants is a good illustra- 

 tion; and many animals are more rhythmically rejuvenescent than 

 others. 



The regenerative capacity must be linked on (i) to the general 

 growth-tendency, the tendency to repeating the same molecules 

 in similar circumstances; (2) to the process of ordinary indirect 

 cell-division, which secures a continuance of the whole inheritance 

 as far as that is carried by the genes in the chromosomes; and (3) 

 to the frequently expressed tendency to developmental recapitula- 

 tion, and it is noteworthy that some regenerations are arrested at 

 a stage either ancestral or embryonic. 



Since embryonic development is the differentiation of a mass of 

 cells, produced from the division of the egg-cell and of its daughter- 

 cells or blastomeres, and since these are early differentiated into the 

 three germinal layers — ectoderm, mesoderm, and endoderm — there 

 cannot be that simple correspondence between normal ontogeny 

 and regeneration which at first sight appeared to the early students 

 of this problem. For the regeneration of a lost part is taking place 

 in some injured corner of a finished organism, and thus must start 

 from a focus which is very different from an embryonic blastoderm. 

 Thus it may be entirely ectodermic ! 



These are some of the considerations that lead us to regard the 

 replacement of a lost part as less surprising than it at first sight 

 seems. But the difficulty of its sporadic occurrences requires to be 

 met in another way, and we agree with Weismann in interpreting 

 the distribution of the regenerative capacity as adaptive. He general- 

 ised and vindicated "Lessona's Law", that regeneration tends to 

 occur in those organisms and in those parts of organisms which are 

 in the natural conditions of their life particularly liable to non-fatal 

 injury. In Weismann's words, "the power of regeneration possessed 

 by an animal or by a part of an animal is regulated by adaptation 

 to the frequency of loss and to the extent of the damage caused by 

 the loss". 



LIFE-HISTORIES 



THE CURVE OF LIFE.— An individual Hfe has often been com- 

 pared to an arched bridge, rising to a short level middle stretch, 

 and then descending again. It is comparable to an ascending and 

 a descending curve — a trajectory. For whether we think of plant 

 or animal, of bird or man, there is a time of developing, growing, 

 strengthening, mellowing; and then comes the time of decline. 



