DEVELOPMENT. 207 



organs. For a time, indeed, the human embryo so closely 

 resembles that of the lower forms as to be indistinguisha- 

 ble from them ; but certain sti'uctures belonging to those 

 forms are kept long after the embryo is clearly human."' 

 All the members of a group do not reach the same degree 

 of perfection, some remaining in what corresponds to the 

 immature stages of the higher animals. Such may be 

 called permanently embryonic forms. 



Sometimes an embryo develops an organ in a rudimen- 

 tary condition, which is lost or useless in tlie adult. Thus, 

 the Greenland Whale, when grown up, has not a tooth in 

 . its head, while in the embryo life it has teeth in both 

 jaws; unborn Calves have canines and upper incisors; 

 and the female Dugong has tusks which never cut the 

 gum. The "splint-bones" in the Horse's foot are unfin- 

 ished metatarsals. 



Animals differ widely in the degree of development 



reached at ovulation and at birth. The eggs of Frogs 



are laid when they can hardly be said to have become 



fully formed as eggs. The eggs of Birds are laid when 



segmentation is complete, while the eggs of Mammals are 



retained by the parent till after the egg-stage is, passed."' 



Ruminants and terrestrial Birds are born with tlie power 



of sight and locomotion. Most Carnivores, Eodents, and 



perching Birds come into the world blind and helpless ; 



while the human infant is dependent for a much longer 



time. 



1. Metamorphosis. 



Few animals come forth from the egg in perfect condi- 

 tion. The vast majority pass through a great variety of 

 forms before reaching maturity. These metamorphoses 

 (which are merely periods of growth) are not peculiar to 

 Insects, though more apparent in them. Man himself is 

 developed on the same general principles as the Butterfly, 

 but the transformations are concealed from view. The 



