TAXONOMY, ANATOMY, EMBRYOLOGY 



recurved against the ventral part of the thorax, the abdominal appendages 

 are much reduced or missing entirely, excepting the first two which serve 

 as copulatory organs. But through all of this great range of variation, a 

 single pattern is discernible, a fact which bespeaks an organic relationship 

 of all of the Crustacea. 



Adaptive Radiation in the Forelimbs of Mammals. The same prin- 

 ciple holds true within each class. The forelimb of the mammals may be 

 taken as an example. In any, there is a single long bone, the humerus, 

 in the upper arm. In the forearm there are two parallel bones, the ulna 

 and the radius. In the wrist there are typically eight carpal bones arranged 

 as two rows of four. Five parallel metacarpals form the skeleton of the 

 palm of the hand, while rows of three phalanges each form the skeleton 

 of the digits, excepting the first digit, which has only two phalanges. 



The shrews ( order Insectivora, family Soricidae ) show a very primitive 

 arm structure (Figure 15). Their close relatives, the moles (family Tal- 

 pidae ) are, however, highly modified for digging. All of the bones of the 

 limbs are short and broad, giving the limb a shovel-like appearance. Thus 

 adaptation is attained by mutual fitting of structure ( the shovel-like limb ) , 

 function (digging), and environment (the subterranean habitat). In the 

 order Chiroptera (bats), the humerus, radius and ulna, and four of the 

 digits are greatly elongated to support the wing membrane. In the ungu- 

 lates, the humerus is short and heavy. The remaining bones of the fore- 

 limb are generally elongated, and the digits are reduced in number. 

 Fusion of bones is quite common in adults, but in the embryos the primi- 

 tive centers of ossification can be identified. The details naturally differ 

 considerably among the various orders of ungulates. 



Examples could be multiplied indefinitely, but the principle remains the 

 same throughout. Within any taxonomic category, all of the members 

 appear to be built upon a common plan, with variation among the various 

 members resulting in the adaptation of each to its mode of life. The higher 

 the category examined, the greater the scope of variation. But the common 

 plan is always discernible. To some of the predecessors of Darwin, this 

 indicated the supernal archetypes. But since Darwin's time, the great 

 majority of biologists have been convinced that close anatomical similarity 

 must be based upon close genetic relationship, while more remote resem- 

 blances are based upon more remote blood relationship. 



Homology and Analogy Contrasted. Another highly suggestive as- 

 pect of comparative anatomy is the comparison of homologous structures 

 which, within a single group, are used for quite different purposes, and 

 analogous structures, which, although quite different morphologically, 

 and developed in different groups, have nonetheless a certain similarity 

 which is based upon adaptation to the same function. If each structure 

 were created for the purpose for which it is now used, analogy should be 

 far more pervasive and important than homology. A few examples will 

 show that this is not the case. A classical example is furnished by the fins 

 of fishes and the flippers of aquatic mammals, such as whales and seals. 

 These structures, characterizing organisms at opposite ends of the verte- 

 brate series, serve the same function. Superficially, they show a close 



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