THE VEGETABLE AND ANIMAL KINGDOMS. 131 



cable to special parts of the organization, as the circulatory 

 and nervous systems, are presented in the diagrams annexed, 

 Figs. 74 and 75. 



FIG. 74. 



BAG 



Diagram of the Principal Forms of the Heart : 



A, Heart of Fish ; a, single auricle ; 6, single ventricle : B, Heart of 

 Reptile; a, right auricle; b, left auricle; c, single ventricle: C, 

 Heart of Mammal; a, right auricle ; b, left auricle; c, right ven- 

 tricle ; c*, left ventricle : D, embryonic heart of Mammal, corre- 

 sponding in development with that of Fish. 



There are even cases in which the embryo of the higher 

 animal may be said to pass through a comparatively mature 

 condition of a grade beneath it. This happens where an 

 animal comes forth from the egg before its development is 

 completed, but in a condition that fits it for an independent 

 existence. We have notable examples in the insects, whose 

 young first appear in the inferior annelidan form (larva), and 

 in the batrachian reptiles, whose progeny are fish for the first 

 few weeks of life (tadpoles). It is not, indeed, pretended that 

 the larva of the insect is a perfect annelid, or that the tadpole 

 is a perfect fish. There is an embryonic character in both cases. 

 Still they are " essentially" annelid and fish. 



When we trace the history of what has been called " the 

 general life of the globe," as far as that has been ascertained by 

 geological research, we find something very like the same plan 

 as that which is followed in " the individual life of every one 

 of the forms of organized being which now people it." (Car- 



