CHAPTER VII 



YOUNG BIRDS AND THE RECORDS OF THE PAST 



Between young birds and young mammals there is not 

 so wide a difference as is commonly supposed, inasmuch 

 as the young echidna and the young platypus are both 

 hatched out from eggs. But while these stand alone in 

 this respect among mammals, it is the universal rule 

 among the birds. All young mammals, however, are 

 suckled by their parents on the peculiar secretion known 

 as milk ; the food of young birds, on the other hand, is 

 furnished by external sources. ' 



With the majority of mammals the true egg, or germ 

 out of which the growing body is developed, is excessively 

 minute, and rapidly becomes attached to the uterine 

 wall of the parent's body, extracting nourishment from 

 the parental tissues. With the young bird the egg or 

 germ is no less minute, but instead of remaining within 

 the parental body and drawing nourishment therefrom, 

 it becomes enclosed within a hard shell together with a 

 relatively enormous mass of yolk, which it slowly absorbs. 

 The growth stages up to the moment of hatching are the 

 embryonic stages, those thereafter are post-embryonic. 



But, as with the mammals, we can draw no hard-and- 

 fast line between embryonic and post-embryonic : and 

 this fact is perhaps more easily grasped in the case of 

 the birds. As everybody knows, young sparrows and 



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