Aim and Method of Comparative Anatomy 371 



organs. Organs which are not homologous may correspond in function 

 and structure and therefore appear deceptively like homologs. It is 

 precisely these obscured homologies and misleading semblances of 

 homology that are most important because it is they that most posi- 

 tively indicate radical evolutionary change and genetic distance be- 

 tween the animals concerned. Therefore the anatomist's first and most 

 difficult task is to discover the hidden homologies and to expose the 

 falsity of the pseudohomologies. His all-important question is, how may 

 homology be recognized? 



There is no single and infallible criterion of homology. Each case 

 must be considered on its own merits. The germ-plasm is continuous 

 only from its present into the future. Its past is marked only by a trail 

 of discontinuous fragments of the organisms which it produced in past 

 thousands and millions of years. The animal lying on the dissecting 

 table today is merely a recent and more nearly complete relic of the 

 germ-plasm. It is true, however, that the experience of nearly a century 

 of evolutionary morphology has given us a little appreciation of the 

 relative values of our comparisons. 



Comparison of two animals requires that the animals be dismem- 

 bered and compared organ by organ. Similarly, two organs cannot be 

 compared immediately as wholes. They must be resolved into their 

 several characteristics, which may then be considered one at a time. 

 Experience teaches that some characteristics are much more significant 

 than others in the testing of homologies. Color is of little, if any, 

 significance. It is true that spleens are usually deep red and a pancreas 

 is yellowish or browivbut these colors are merely incidents of difference 

 in the underlying structure. The spleen is more richly filled with blood- 

 vessels. Size has merely functional significance. The hearts of mouse 

 and elephant are certainly homologous. 



Number of such multiple parts as vertebrae and muscle segments 

 is usually of little significance. Animals which are certainly very closely 

 related may have quite different numbers of myomeres, vertebrae, and 

 ribs. Modern sharks have six, seven, or eight pairs of pharyngeal gill- 

 clefts. In some organs, however, number seems to have become firmly 

 fixed — the paired appendages, for example. When we find a vertebrate 

 with only one pair or none at all, we are confident that the deficient 

 animals are genetically far removed from ordinary four-finned or four- 

 legged vertebrates. In land vertebrates the number of major jointed 

 segments arranged successively along the main axis of the appendage 

 is fixed. Shoulder girdle, upper arm, forearm, wrist, and hand corre- 

 spond respectively to hip girdle, thigh, shank, ankle, and foot. These 

 divisions remain numerically constant whether the appendage is the 

 foreleg of a giraffe, the hindleg of a kangaroo, or any leg of a dachshund. 



