34 THE DOCTRINE OF DESCENT. 



characteristics of form and internal structure ; and it is 

 very easy to select from every type forms in which the 

 distinctive marks, comprised in the systematic diagnosis, 

 may be displayed in full perfection. But this is imme- 

 diately succeeded by a further observation, that of gra- 

 dations within the type. When we previously compared 

 the polype and the bee, and were obliged to assign to 

 each a very different rank, a portion of this difference 

 of grade is certainly due to the difference of the family; 

 but the forms united by family characteristics likewise 

 diverge widely from each other, and the systematist 

 speaks of lower and higher classes within every type, of 

 lower and higher orders within every class. 



Reason is compelled to this by the same considerations 

 which forced themselves upon us in the comparison of the 

 polype and the bee. Why does the mussel stand lower 

 than the snail .^ Because it does not possess a head, 

 because its nervous system is not so concentrated and 

 so voluminous, because its sensory organs are more de- 

 fective. In one, as in the other, the structural material 

 is present in quantities sufficient for the completion of 

 the type ; but in the snail it is more developed, and the 

 single circumstance of the integration of various parts to 

 form the head confers a higher dignity upon the snail. 

 It is needless to illustrate this gradation within the 

 families by further examples ; the most superficial com- 

 parison of a fish with a bird or a mammal, of one of 

 the parasitic Crustacea with a crayfish or an insect, 

 shows, as the older zoology represented it, that in the 

 actual forms the ground plan, or "ideal types," find 

 very diversified expression. 



A further result of this descriptive inquiry is the 



