ADAPTATIONS 



305 



birds, such as the chicken, the duck, and the auk, as with 

 the reptiles, the young animal is hatched with well-devel- 

 oped muscular system and sense 

 organs, and is capable of running 

 about, and, to some extent, of feed- 

 ing itself. Birds of this type are 

 known as prcecocial (Fig. 172), while 

 the name altricial (Fig. 173) is ap- 

 plied to the more highly organized 

 forms, such as the thrushes, doves, 

 and song-birds generally. With 

 these the young are hatched in a 

 wholly helpless condition, with in- 

 effective muscles, deficient senses, 

 and dependent wholly upon the 

 parent. The altricial condition de- 

 mands the building of a nest, the 

 establishment of a home, and the 

 continued care of one or both of 



laid) cut open to show young the parents. 



S^^SrJSrJT Theverylowestmammalsknown, 

 the duck-bills (Monotremes) of 



Australia, lay large eggs in a strong shell like those of a 

 turtle, and guard them with great jealousy. But with 

 almost all mammals the egg is very small and without 

 much food-yolk. The egg begins its development within 

 the body. It is nourished by the 

 blood of the mother, and after birth 

 the young is cherished by her, and 

 fed by milk secreted by specialized 

 glands of the skin. All these features 

 are adaptations tending toward the 

 preservation of the young. In the 

 division of mammals next lowest to the Monotremes the 

 kangaroo, opossum, etc. the young are born in a very im- 

 mature state and are at once seized by the mother and 





FIG. 176. Egg-case of the cock- 

 roach. 



