298 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1940 



long, gossamer thread that offers so much resistance to the wind 

 and is so light that they can let go of their support, cling to their 

 strand and be transported considerable distances by the breezes. 

 They are, of course, unable to choose their direction other than as 

 they may let go when the prevailing wind is in the direction that 

 they desire to travel. 



CARE OF YOUNG 



Both the extent and the methods of caring for the young vary as 

 widely as do other animal characteristics. No two species follow 

 exactly the same methods. Some give painstaking individual care to 

 their babies over long periods. Others look after the young for only 

 a brief period until the precocious youngsters learn to shift for them- 

 selves. The rabbits are good examples of this second type. Still 

 others give their young no care whatever. 



The most primitive of the mammals, the platypus {Or-nitho?-- 

 hynchus) of Australia and the so-called spiny anteaters iEchidna 

 and Zaglossus) of Australia and Tasmania lay eggs. When the 

 young are hatched they obtain the mother's milk by licking the skin 

 on the surface of her chest, for these animals lack nipples. 



The next higher group of mammals are the marsupials. Most 

 marsupials have a very short gestation period and their young are 

 born in a very incompletely developed state. The large kangaroos 

 that stand as high as a man and weigh almost as much, give birth 

 to young that weigh only a fraction of an ounce, after a gestation 

 period of certainly less than 50 days and probably only about 20 or 

 30 days. The little one immediately after birth crawls into the 

 abdominal pouch of the mother and remains there continuously 

 for 3 or 4 months. After that, for a month or two, it spends a por- 

 tion of its time in the pouch and a portion of its time moving about 

 near its mother. If danger threatens, it quickly takes shelter in 

 the pouch. Not all mammals that belong to the group known as 

 marsupials have pouches. Those that lack the pouches give birth 

 to young that are somewhat better developed than those of the 

 pouched kinds. These young ones are cared for in a nest. 



At the other extreme from the pouched mammals is the elephant, 

 which has a gestation period of 18 to 21 months. The young ele- 

 phant when born weighs about 220 pounds. It is carefully tended 

 and nursed by its mother during a period that lasts from 2 to 3 

 years. 



The monkeys also have a long gestation period. "Wliile the young 

 at birth are fully formed they are almost as helpless as new-bom 

 human babies. They are carefully tended by their mothers for 

 many months. 



