VI. Biology. 5. The Stages of Life. — Duration of Life. 69 



In each year and in each stage of life there are, often, regularly 

 recurring periods. For many animals, there is, daily, a period of 

 activity and a period of rest. Many sleep during the latter, i.e., 

 they fall into a peculiar state of unconsciousness, in which the 

 activity of the organs is actually lessened (Mammals, Birds). The 

 time of rest usually occurs in the night, the period of activity in the 

 day ; but it is well known that some rest in the day, whilst they 

 become energetic in the evening or at night (nocturnal animals). 



The year is similarly divided for very many animals into two 

 great periods, one of which is devoted to active life, during which 

 the daily activity and rest may naturally alternate : the other 

 to repose. This is specially the case with animals of the temperate 

 and frigid zones, which are, for the most part, rendered entirely 

 passive by the cold of winter, when their vitality is reduced to a 

 minimum (e.gf., hibernating insects). Many Mammals (Bears, Dormice) 

 fall during this time into "winter sleep" (hibernation), a state 

 resembling ordinary sleep, but of an intensified kind. The un- 

 consciousness is deeper, the activity of the organs decidedly 

 less, and the temperature of the body may fall several degrees. 

 Similarly the resting period of tropical forms may occur during the 

 dry season, and in certain English animals the hottest summer- 

 time may occasion a summer sleep (Earthworms, Eeptiles, 

 Amphibia) . 



Many animals, especially those of cold climates, have an annual 

 breeding season, at which time the ova and spermatozoa are 

 ripe, and when pairing takes place. At other times the ovaries and 

 testes are in a state of relative inactivity, and the sexual impulse is 

 extinguished. In warm countries, at least in most cases, such a 

 period does not occur. Many animals, whose nearest relatives in the 

 temperate zone have a limited period for breeding, reproduce all the 

 year round. It must also be noticed that the periodicity of marine 

 forms is less pronounced than that of land animals ; ripe ova are 

 found in some marine animals throughout the year in cold regions. 

 This diversity naturally depends on the fact that the variations of 

 temperature are much less in sea-water than in air. 



Death, i.e, the final cessation of all manifestations of life, 

 usually occurs in consequence of the functional passivity of a vital 

 organ." If, for instance, the heart of a Vertebrate ceases to contract, 

 the body is deprived of a necessary condition of existence, viz., the 

 supply of blood laden with oxygen, and all the organs and tissues 

 gradually die. An exact moment of death cannot be spoken 

 of : if a Mammal be killed by a violent blow on the head, move- 

 ments of the heart and respiratory organs, of course, cease abnost 

 immediately, and the animal is said to be dead : but many of the 



