12 THE BOOK OF NATURE STUDY 



II. To care for their offspring. 



Do they lay eggs ? If so, where, when, and how ? Do 

 they bring forth young ones as such ? Are these 

 miniatures of the parents, or are they at first very 

 unlike the parents ? How do the parents secure the 

 safety of their eggs or young ones ? How many 

 do they produce at one time ? Do they feed their 

 young, guard them, tend them, teach them ? For 

 how long do the young ones remain immature ? 

 What are the relations between the two parents, if 

 there are two parents ? Do both help to care for 

 the young ? 



SECTION 4. THE PERIODS OF LIFE. Every organism repro- 

 duced in the ordinary way begins its individual life as a fertilised 

 egg-cell. This divides and redivides, the resulting cells begin 

 to grow and continue to divide, they group themselves in layers 

 which are variously pouched and folded, division of labour sets 

 in, and apparent simplicity is replaced by obvious complexity. 

 We speak of the developing animal as an embryo, until it is 

 " hatched/' i.e. freed from the egg envelopes. When the young 

 creature that emerges from the egg is very unlike the parent 

 built on a different architectural plan it is called a larva, and 

 in such cases (e.g. caterpillars, tadpoles) there has to be a pro- 

 found structural change (a metamorphosis) before the distinctive 

 characters of the fully formed type begin to be assumed. After 

 these are attained there is a longer or shorter period of growth 

 or adolescence, which continues until the sexual maturity is 

 attained. In the majority of cases the advent of reproductive 

 power is associated with the limit of growth, that is, with reaching 

 a definite size probably the most useful and effective size for 

 the type. Then follows a longer and shorter period of maturity, 

 of full vigour both in work and reproduction. Sooner or later 

 the arrears of bodily wear and tear and the expenses of repro- 

 duction begin to tell, waste is no longer quite made good by 

 repair, and the animal passes into senescence, ending in death. 

 It is useful to try to make curves expressing in diagrammatic 

 form the different kinds of life-histories. There may be a long 



