422 ORIGINATIVE FACTORS IN EVOLUTION: 



Is it not possible that some similar re-organisation may 

 normally occur in Metazoa at the origin of each individual 

 life, and that, if it does, there is no need to look about for 

 any special cause ? It is all in the day's work, it is part of 

 the programme of the essentially regulative life-cycle. We 

 may recall, too, that variation occasionally occurs in 

 parthenogenetic or aspermic development, as well as in the 

 ordinary process. 



We are not seeking to ' explain ' variations by verbal 

 inventions. Our argument is quite clear: Certain mutations 

 in organisms are preceded by germinal disturbances, perhaps 

 these germinal disturbances are comparable to endomixis in 

 Paramecium. It is always a step towards understanding 

 to put one obscure process alongside of another which is 

 similar to it and which may be more amenable to experi- 

 mental treatment. Therefore we suggest that endomixis may 

 be profitably considered along with the problem of the origin 

 of variations. 



Another gleam of light may possibly be found in Professor 

 Child's long-continued study of processes of senescence 

 and rejuvenescence, a study recently presented in its 

 entirety in a remarkable volume Senescence and Rejuvenes- 

 cence (1915). Professor Child finds that when a fragment 

 of a Planarian regrows a whole, there is a rejuvenescence dur- 

 ing the re-constitution ; the rate of metabolism is high and the 

 resistance-power is great. The metabolism may be measured 

 by Tashiro's ' biometer ', an extraordinarily delicate reg- 

 ister of the CO 2 output, or more indirectly by the degree 

 of susceptibility and resistance to cyanide poisons and the 

 like. Judged by these tests, the regenerating piece of Plana- 

 rian is younger than it was when it formed part of the parent. 

 It literally renews its youth. Similarly, when a Planarian 



