CHAPTER 21 • INFANCY AND PARENTHOOD 



1 Why is a cat more helpless at birth than a calf? 



2 How does an animal benefit by looking after its young? 



3 Why do not all animals take care of their young? 



4 Against what do the young of plants and animals have to be pro- 



tected? 



5 Why do some species produce such tremendous numbers of eggs 



or seeds? 



6 Can anything be done to hasten or to slow up the d£velopment of 



a plant or an animal? 



7 Can a kitten's development be hurried by forcing its eyes open? 



8 What makes a hen want to sit on the eggs at one time, but not at 



another? 



Each plant and each animal typically starts life as a single cell. When a 

 one-celled plant or animal reproduces itself, it gives up absolutely everything — 

 its own individual existence — to the offspring. Young and old are much aUke; 

 the new individual at once starts out on its own. 



In more complex organisms the initial cell is usually a spore or a zygote; 

 and the initial stage is in every way different from the adult. Among the 

 many-celled species the individual that reproduces normally holds on to life, 

 but the new individual is helpless and dependent. 



In what ways are the more complex species better off than the simpler ones? 

 Why do we call them higher? In what ways are the simplest organisms less 

 capable of surviving? 



Why Do We Consider Some Forms of Life Higher than Others? 



Lines of Differentiation Even among the lower classes of many-celled 

 plants and animals, speciaHzation of function is already beginning. And there 

 is a corresponding specialization of structures or of organs. In the hydra, for 

 example, the outer and the inner cell layers behave differently in relation to 

 external stimuU and in relation to food; the middle layer gives rise to 

 reproductive cells (see illustration, p. 274). The cells grow and divide, as 

 in one-celled animals, according to the food supply and other conditions. 

 But the whole individual continues over a much longer period. The longer 

 span of life means more development, more ways of getting about, more ways 

 of getting food — and more dangers to run into, too. 



The earliest division of labor in the history of life is probably that between 

 food-getting and food-using, as in the hydra. We might even go farther back 

 and think of the entire plant world and the entire animal world as distinct 



417 



