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The biological science of the last half-century is honourably 

 distinguished from that of preceding epochs, by the constantly 

 increasing prominence of the idea, that a community of plan is dis- 

 cernible amidst the manifold diversities of organic structure. That 

 there is nothing really aberrant in nature ; that the most widely dif- 

 ferent organisms are connected by a hidden bond ; that an appa- 

 rently new and isolated structure will prove, when its characters 

 are thoroughly sifted, to be only a modification of something which 

 existed before, are propositions which are gradually assuming the 

 position of articles of faith in the mind of the investigators of ani- 

 mated nature, and are directly, or by implication, admitted among 

 the axioms of natural history. 



And this is not wonderful ; for no living being can be attentively 

 studied without bearing witness to the truth of these propositions. 

 The tyro in comparative anatomy cannot fail to be struck with the 

 resemblances between the leg and the jaw of a crustacean ; between 

 the parts of the mouth of a beetle and those of a bee ; between the 

 wing of the bird and the fore-limb of the mammal. Everywhere he 

 finds unity of plan, diversity of execution. 



Or again, how can the intelligent student of the human frame con- 

 sider the backbone, with its numerous joints or vertebra, and trace 

 the gradual modification which these undergo downwards into the 

 sacrum and coccyx, and upwards into the atlas and axis, without 

 the notion of a vertebra in the abstract, as it were, gradually dawn- 

 ing upon his mind ; the conception of an ideal something which shall 

 be a sort of mean between these various actual, forms, each of which 

 may then easily be conceived as a modification of the abstract or 

 typical vertebra? 



^ / Such an idea, once clearly apprehended, will hardly permit the 

 mind which it informs to rest at this point. A glance at a section 

 of that complex bony box formed by the human skull and face, 

 shows that it consists of a strong central mass, whence spring an 

 upper arch and a lower arch. The upper arch is formed by the 

 walls of the cavity containing the brain, and stands in the same re- 

 lation to it, as does the neural arch of a vertebra to the spinal cord, 

 with which that brain is continuous. The lower arch encloses the 

 other viscera of the head, in the same way as the ribs embrace those 

 of the thorax. And not only is the general analogy between the 



