392 THE INDUCTIONS OF BIOLOGY. 



apparatus — a nervous system and a muscular system. But 

 these higher sub-kingdoms are clearly separated from one 

 another by differences in the relative positions of their com- 

 ponent sets of organs. The habitual attitudes of annulose 

 and molluscous creatures, is such that the neural centres are 

 below the alimentary canal and the haemal centres above. 

 And while by these traits the annulose and molluscous types 

 are separated from the vertebrate, they are separated from 

 each other by this, that in the one the body is " composed of 

 successive segments, usually provided with limbs," but in the 

 other, the body is not segmented, " and no true articulated 

 limbs are ever developed.'' 



The sub-kingdoms being thus distinguished from one an- 

 other, by the presence or absence of specialized parts devoted 

 to fundamental functions, or else by differences in the distri- 

 butions of such parts, we find, on descending to the classes, 

 that these are distinguished from one another, either by 

 modifications in the structures of fundamental parts, or by 

 the presence or absence of subsidiary parts, or by both. 

 Fishes and Amphibia are unlike higher vertebrates in possess- 

 ing branchiae, either throughout life or early in life. And 

 every higher vertebrate, besides having lungs, is characterized 

 by having, during development, an amnion and an allantois. 

 Mammals, again, are marked off from Birds and Reptiles by 

 the presence of mammae, as well as by the form of the 

 occipital condyles. Among Mammals, the next division is 

 based on the presence or absence of a placenta. And divisions 

 of the Placentalia are mainly determined by the characters 

 of the organs of external action. 



Thus, without multiplying illustrations and without de- 

 scending to genera and species, we see that, speaking gener- 

 ally, the successively smaller groups are distinguished from 

 one another by traits of successively less importance, physio- 

 logically considered. The attributes possessed in common 

 by the largest assemblages of organisms, are few in number 

 but all-essential in kind. Each secondary assemblage, in- 



