The Story of Evolution 71 



about the development of an antler or a wattle or a decorative 

 plume or a capacity for vocal and saltatory display. In some 

 cases it is certain that the female carries in a latent state the mas- 

 culine features, but these are kept from expressing themselves by 

 other chemical messengers from the ovary. Of these chemical 

 messengers more must be said later on. 



Recent research has shown that while the difference between 

 male and female is very deep-rooted, corresponding to a differ- 

 ence in gearing, it is not always clear-cut. Thus a hen-pigeon 

 may be very masculine, and a cock-pigeon very feminine. The 

 difference is in degree, not in kind. 



5 



What is the meaning of the universal or almost universal 

 inevitableness of death? A Sequoia or "Big Tree" of California 

 has been known to live for over two thousand years, but eventu- 

 ally it died. A centenarian tortoise has been known, and a sea- 

 anemone sixty years of age; but eventually they die. What is 

 the meaning of this apparently inevitable stoppage of bodily 

 life? 



The Beginning of Natural Death 



There are three chief kinds of death, (a) The great 

 majority of animals come to a violent end, being devoured by 

 others or killed by sudden and extreme changes in their surround- 

 ings, (b) When an animal enters a new habitat, or comes into 

 new associations with other organisms, it may be invaded by a 

 microbe or by some larger parasite to which it is unaccustomed 

 and to which it can offer no resistance. With many parasites a 

 "live-and-let-live" compromise is arrived at, but new parasites 

 are apt to be fatal, as man knows to his cost when he is bitten by 

 a tse-tse fly which infects him with the microscopic animal (a 

 Trypanosome) that causes Sleeping Sickness. In many animals 

 the parasites are not troublesome as long as the host is vigorous, 



