THE CYCLE OF LIFE 439 



in virtue of some constitutional toughness survive longer 

 and have more offspring. As Dr. Chalmers Mitchell 

 points out, however, the process might work the other 

 way round by a selection of those variants showing 

 increased reproductivity. If the specific duration of life 

 happened to be a very fixed character, and the fertility 

 very variable, the line of solution might be as Dr. Chalmers 

 Mitchell indicates. Both theories may be right. Unfortun- 

 ately, neither admits of verification as regards the past. 



Death. In spite of criticisms, we find no good reason 

 against accepting Weismann's doctrine of the immor- 

 tality of the Protozoa. Truly, these simple organisms 

 do not live a charmed life ; they are continually being 

 killed in countless millions ; they are sometimes consumed 

 by parasites, and so on ; but the point is that some of 

 them at least are not subject to natural death in the same 

 degree as higher animals are ; that some of them, indeed, 

 may be exempt from natural death altogether. To be 

 devoured by other creatures, to be dried up by the sun, 

 to be killed by a sudden change of temperature, that is the 

 fate of many ; but that is violent death. Others are 

 occasionally destroyed by internal parasites smaller and 

 simpler than themselves, but that is microbic death. To 

 the natural death which ensues from the physiological 

 insolvency of the body they are immune. The reasons 

 are to be found in their relative simplicity of structure ; 

 they can continuously make good their wear and tear ; 

 and in their relatively simple modes of multiplication, 

 which do not involve the nemesis so familiar in higher 

 animals. It is well known that a family of Infusorians all 

 descended from one individual isolated in a basin will often 

 come to an end, one of the reasons being the absence of 



