GREAT STEPS IN EVOLUTION 87 



But out of this weakness if weakness 

 strength arose, the strength of animals 

 with a body. 



BEGINNING or DEATH. In a startling 

 phrase the immortality of the Protozoa 

 Weismann called attention to the fact that 

 unicellular organisms are not subject to 

 natural death in the same degree as higher 

 animals are. They may be killed, of course, 

 in many ways, but they do not normally die. 

 Even against microbic infection many of them 

 seem proof; they digest the virulent intruders, 

 as do the phagocytes which form our body- 

 guard. But the point is, that in natural 

 conditions, where inter-crossing, for instance, 

 is readily feasible, they appear to be exempt 

 from that natural death which in the higher 

 organisms is due to the slow mounting-up of 

 physiological arrears. 



How is it that these simple pioneer organ- 

 isms are exempt from the penalty all other 

 flesh is heir to ? The answer is twofold. On 

 the one hand, being relatively very simple, 

 in a strict sense without a " body " they are 

 able to sustain with persistent success the 

 vital equation between waste and repair. On 

 the other hand, their common mode of repro- 

 duction, by dividing into two or more units, 

 is inexpensive and not attended with any loss 



