Originative Factors in Evolution. By J. A. Thomson. 44S 



conjugation. For conjugation is not going to occur ; something 

 that takes its place is occurring — endomixis. Now, it seems 

 probable that such a periodic reoganization of nuclear material 

 will afford opportunity for plasmic rearrangement, and that may 

 imply the origin of variations even within a pure line. Jennings 

 Jias found in pure lines of non-conjugating Paramecium evidence 

 of variations about the mean. These might be due to rearrange- 

 ments effected in endomixis. It is conceivable, as Woodward and 

 Erdmann point out, that " heritable " variations may result from 

 some rare recombinations in endomixis. 



This Paramecium is a very complicated organism, as Clifford 

 Dobell has vividly emphasized, on the non-cellular line of evolu- 

 tion, and we tind it in certain conditions exhibiting a monthly 

 reorganization as part of its life-cycle. Is it not possible that 

 some similar reorganization 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 may occur in parthenogenetic 

 ■or aspermic development, as well as in the ordinary process. 



We are not seeking to " explain " variations by verbal inven- 

 tions. 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 experimental treatment. Therefore we 

 suggest that endomixis may be profitably considered along with 

 the problem of the origin of variations. 



(d) Another gleam of light may possibly be found in Child's 

 long-continued study of processes of senescence and rejuvenescence 

 — a study recently presented in its entirety in a remarkable volume, 

 " Senescence and Eejuvenescence " (1915). Child finds that when 

 a fragment of a Planarian regrows a whole, there is a re- 

 juvenescence during 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 

 register of the CO.2 output, or more indirectly by the degree of 

 susceptibility and resistance to cyanides and the like. Judged by 

 these tests, the regenerating piece of Planarian is younger than it 

 was when it formed part of the parent. It literally renews its 

 youth. Similarly, when a Planarian or a Hydroid multiplies 

 asexually, the separated-off piece shows marked rejuvenescence, as 

 revealed by the two tests named. 



Child's thesis is this : As an organism differentiates, it ages, for 

 the accumulation of relatively inactive constituents in the colloidal 

 •cytoplasmic substratum necessarily involves a decrease in the 



