70 General Part. 



tissues continue to live for many hours tlie muscles are capable 

 of contraction, etc. 



Death may occur in any period of life : in forms which produce 

 very numerous ova, the majority of specimens invariably die in the 

 embryonic or larval stages, only a small proportion attaining sexual 

 maturity. This results almost always from detrimental influences 

 in the environment. The existence of most animals terminates 

 because they are killed or eaten by others : many fall victims 

 to pathogenic parasites, principally Bacteria and Fungi; others, 

 again, succumb to the effects of climatic changes. Death, 

 conditioned by the normal internal state of the body, occurs rarely : 

 even when an animal appears to die of old age, or when, as is 

 frequently the case, it dies, necessarily, after a single production 

 and discharge of ova or spermatozoa, external conditions may 

 still be an agent. 



The duration of life in animals, the time over which life 

 can extend under favourable circumstances, vaTies enormously 

 for different forms : in some, it may be limited to a few weeks or 

 less, in others it may extend to a hundred years or more. The 

 rule may be generally laid down that in a natural group, the 

 larger species live longer than the smaller, just as their develop- 

 mental period is longer : the Elephant lives more than a hundred 

 years, the Horse, seldom more than thirty ; the Mouse, only a few ; 

 Large Insects often live several (the Cockchafer, e.g., four), smaller 

 ones, a year, or even a few months only. In some, the duration of 

 life is very definite {e.g., most Insects die soon after laying their 

 eggs), amongst others it is quite unlimited. 



6. Protective Adaptations. 



It is well-known that most animals are preyed upon by others, 

 and, as a rule, they do not confront the enemy defenceless, but are 

 protected in different ways. 



The means of protection are sometimes offensive: the prey 

 itself practically attacks the aggressor. The weapons used in defence 

 are sometimes such as are used in other cases for attack : a carnivore, 

 for instance, tears its prey to death with its teeth, and also uses them 

 as defensive weapons when it is attacked. As a rule, animals resist 

 their enemies with any means at their disposal; Mammals with 

 hard hoofs, kick; if provided with strong claws, they scratch; 

 many insect larvge pour out a malodorous or sticky secretion. The 

 caudal spines of some Rays ; the stink-glands of many Insects and 

 of the Skunk {Mephitis), whose disagreeable secretion is ejected to 

 drive away the enemy, are special weapons of defence. 



Other animals try to frighten the enemy by violent cries, 

 by terrifying movements, by bristling the feathers or hair. 



