Aim and Method of Comparative Anatomy 359 



mechanisms into their anatomic elements, we discover some resem- 

 blance between the spiracle and certain cavities of the ear. Turning to 

 the embryos, we find that the most anterior pharyngeal pouches be- 

 come spiracles in a fish and "middle-ear" cavities in a mammal, while 

 certain embryonic parts in a mammal produce two of the three ear- 

 bones and corresponding parts in a reptilian embryo produce the two 

 bones which serve as the members of the joint between the upper and 

 lower jaw. 



It would seem that a kidney should correspond to a kidney. As 

 organs of excretion, they correspond functionally. If a mammalian 

 kidney is compared with that of a fish or amphibian, it will be found 

 that there is general similarity in structure, although there are some 

 differences. But in their embryonic histories they do not correspond. 

 As to the position and source of the embryonic kidney-forming ma- 

 terial, and in maimer of development, they are very different. Further, 

 the mammalian embryo produces temporary renal structures which cor- 

 respond anatomically to the kidneys of fishes and amphibians. 



Lungs and gills correspond in function. In gross anatomy they are 

 totally different. In bodily position they are far apart, but a point of 

 resemblance is found in the fact that gill-chambers open into the 

 pharynx and lungs connect with the pharynx by way of the trachea 

 ("windpipe"). In the embryo lateral pharyngeal pouches produce gill- 

 chambers, and a single median ventral outgrowth of the wall of the 

 pharynx produces lungs. 



In the foregoing comparisons we have encountered correspondences 

 in function, position, structure, and embryonic origin. But, for the 

 purposes of comparative anatomy, confusion arises from the fact that 

 two organs may correspond in one or more of these particulars but not 

 in all of them. Adult organs (e.g., kidneys) which are so similar that 

 they would seem to correspond may have very different embryonic 

 origins. Corresponding embryonic parts may produce organs which, 

 in the adults, are totally unlike in structure and function. The vital 

 question comes to be, then, what kind of correspondence is relevant to 

 our purpose? 



If an animal be compared with one of its parents or grandparents, 

 there is no confusion about correspondence — unless some major muta- 

 tion has occurred. Organs which seem to correspond do correspond, 

 brain to brain, eye to eye, heart to heart. We may speak of an organ as 

 being the same organ as the corresponding organ of parent or grand- 

 parent. In the remainder of this discussion, the word "same" will be 

 used to signify correspondence due to inheritance. Evolution, 

 however, implies change. If we were to compare two animals which are 

 many thousands of generations and millions of years apart, we might 



