374 Comparative Anatomy — Its History, Aim, and Method 



ignored but, in general, cellular structure and histologic structure have 

 little or no significance in relation to homology. Types of architecture 

 cannot be classified on the basis of building materials. A Gothic church 

 or a modern office building may be constructed of bricks of the same 

 sort. Skull bones are histologically similar regardless of homologies. 

 Glandular structures of various sorts may be histologically very much 

 alike. But this does not imply that the anatomist need not use his 

 microscope. Many elementary organs are of microscopic size. Renal 

 tubules and hepatic tubules are, respectively, elementary constituent 

 organs of those larger organ complexes, the kidney and liver. Micro- 

 scopic anatomy must be distinguished from histology. 



In contrast to the characteristics mentioned above, position of 

 organs is of great significance — but the comparative anatomist uses 

 that word in a special sense. To him, "position" is not necessarily the 

 place which the organ occupies in the body of the animal. It is relative 

 position or, most especially, position as indicated by the connections of 

 one part to other parts. A century of comparative anatomy affords ample 

 evidence that an organ, once established in certain spatial relations to 

 other organs, strongly tends to retain those relations, however much it 

 may change in function, form, and structure. This "principle of con- 

 nections" was vaguely appreciated by Goethe, elaborated and empha- 

 sized by Geoffroy Saint-Hilaire, recognized by Charles Darwin as 

 highly important, and used by Owen as the chief, or even the only, cri- 

 terion of homology. Gegenbaur and later anatomists admit its value. 



The "principle of connections" may be illustrated by imagining 

 a fleet of ships anchored in a large harbor. Because of the contour of 

 surrounding mountains and irregularities of shore-line and bottom, the 

 winds and currents are not uniform over the entire harbor and they 

 vary considerably in the course of the day. Each ship, riding freely at 

 anchor, swings now this way and now that, depending on the local 

 breeze and tide. The ships do not all swing in the same direction at 

 once. In the afternoon, ship "A" is lying at the spot where ship "B" 

 was in the morning. The pattern of the spots occupied by the several 

 ships in the course of the day changes. But, unless a ship drags anchor, 

 the pattern of the locations of the anchors on the bottom remains the 

 same. Organs are anchored by supporting membranes, ligaments, ten- 

 dons, ducts, nerves, and blood-vessels. (Blood-corpuscles drift freely 

 in the blood-stream, but they are single cells, not anatomic organs.) 

 Subject to the changing winds and tides of function and adaptation, 

 organs may shift from one place to another in the body, but they 

 rarely "drag anchor." If the anatomist can find the anchors, he will 

 not mistake ship "B" for ship "A," nor will he identify the mid- 

 ventral sternum ("breastbone") of land vertebrates with somewhat 



