INADEQUACY OF NATURAL SELECTION, ETC. 637 



negatives further alterations, molar or molecular a state of 

 balance among the forces of the aggregate and the forces which 

 oppose them/" In so far, therefore, as Professor Weismann's 

 conclusions imply the non-necessity of death, they cannot be 

 sustained. 



But now let us consider the above-described antithesis be- 

 tween the immortal Protozoa and the mortal Metazoa, An essen- 

 tial part of the theory is that the Protozoa can go on dividing 

 and subdividing without limit, so long as the fit external condi- 

 tions are maintained. But what is the evidence for this ? Even 

 by Professor Weismann's own admission there is no proof. On 

 p. 285 he says : 



" I could only consent to adopt the hypothesis of rejuvenescence [achieved 

 by conjugation], if it were rendered absolutely certain that reproduction by 

 division could never under any circumstances persist indefinitely. But this 

 cannot be proved with any greater certainty than the converse proposition, 

 and hence, as far as direct proof is concerned, the facts are equally uncertain 

 on both sides." 



But this is an admission which seems to be entirely ignored 

 when there is alleged the contrast between the immortal Protozoa 

 and the mortal Metazoa. Following Professor Weismann's method, 

 it would be " easy to imagine " that occasional conjugation is in 

 all cases essential ; and this easily imagined conclusion might fitly 

 be used to bar out his own. Indeed, considering how commonly 

 conjugation is observed, it may be held difficult to imagine that 

 it can in any cases be dispensed with. Apart from imaginations 

 of either kind, however, here is an acknowledgment that the 

 immortality of Protozoa is not proved ; that the allegation has no 

 better basis than the failure to observe cessation of fission ; and 

 that thus one term of the above antithesis is not a fact, but is 

 only an assumption. 



And now what about the other term of the antithesis the 

 alleged inherent mortality of the somatic cells ? This we shall, I 

 think, find is no more defensible than the other. Such plausi- 

 bility as it possesses disappears when, instead of contemplating 

 the vast assemblage of familiar cases which animals present, we 

 contemplate certain less familiar and unfamiliar cases. By these 

 we are shown that the usual ending of multiplication among 

 somatic cells is due, not to an intrinsic cause, but to extrinsic 

 causes. Let us, however, first look at Professor Weismann's 

 own statements : 



" I have endeavoured to explain death as the result of restriction in the 

 powers of reproduction possessed by the somatic cells, and I have suggested 

 that such restriction may conceivably follow from a limitation in the 



* Sec First Principles, Part II, Chap. XXII, " Equilibration." 



