DISTINCTIVE CHARACTERISTICS. 33 



nutrition and growth of the embryo do not require 

 to be restricted to the comparatively small amount 

 of yolk in the egg, but, as a rule, are derived 

 directly from the blood of the mother. 



We know of the ' quadrate bone ' which in the 

 Mammalia has to a certain extent become dis- 

 placed and slipped into the skull from the Fishes ; 

 we see the beginnings of a diaphragm in the 

 Amphibians ; we find various kinds of skin-glands 

 (to which the lacteal glands belong) in all of the 

 Vertebrates ; there is but one step from the distri- 

 bution of the embryonal blood-vessels on the so- 

 called allantois of reptiles and birds, up to the 

 formation of the placenta : these characteristics of 

 the Mammalia are all prepared or begun in the 

 lower classes of animals. But the placenta came 

 into existence only with the actual mammals, and is 

 an acquisition towards a higher degree of progress. 

 But although these characteristics are obviously 

 inheritances apart from the last-named arrange- 

 ment, which was acquired only subsequently still 

 we are absolutely without any transition forms. It 

 can only be said that the Mammalia must have de- 

 veloped from one stock, where the characteristics 

 of the present Amphibians (for instance, the two 

 occipital condyles at the back of the head) were 



