488 BIOLOGY AND HUMAN LIFE 



between those that leave their eggs as soon as they are laid and 

 those (like the wasps, bees, and ants) that build elaborate nests 

 for them, store away food, and actually nurse and protect them. 



360. Vertebrates. Among all the backboned animals above 

 the amphibians fertilization takes place within the body of the 

 mother. The eggs begin to develop immediately after fertiliza- 

 tion and are retained for a longer or shorter period. Here they 

 are not only protected from possible injury by enemies but are 

 nourished, supplied with moisture, and in many cases kept 

 warm. The eggs of reptiles and birds, as they leave the mother's 

 body, are comparatively large, containing a considerable amount 

 of concentrated food. ]Most reptiles and some birds leave their 

 eggs to be hatched by the heat of the sun or at ordinary tem- 

 peratures, but most of the common birds build more or less 

 elaborate nests and care for the fledglings and for the eggs, 

 besides supplying them with heat for hatching. The feeding of 

 young birds by the parents is a very interesting thing to watch, 

 and it shows a very complex development of instincts. 



Among the mammals, the egg develops inside the body of the 

 mother until it has acquired the general form characteristic of 

 the species, and it is nourished by the parent for a long time 

 after birth. Among the marsupials, or pouch animals, like the 

 kangaroo and the opossum (see Fig. 60 ), the young are placed 

 in an abdominal pouch immediately after birth ; in the other 

 mammals the young suckle from the milk glands of the mother. 

 Passing from the lower mammals to the higher, the infancy of 

 the individual becomes proportionately longer. 



361. Infancy. If we compare the various groups of plants, or the 

 various groups of animals, with respect to the amount of nourishment 

 and protection or any other service that parents render to the young, we 

 find that with the advance of life from the lowest forms to the highest 

 there is an increase of dependence of the offspring upon the parent, and 

 an increased advantage to the species. 



The production of flowers and fruits and seeds, or the production of 

 well-stored eggs, the building of nests, the carrying of the young, all 

 these things mean a great expense to the organism in material and 



