TAXONOMY, ANATOMY, EMBRYOLOGY 



less specialized than the others. These may be known only as fossils, but 

 many such archaic or primitive species are still living. In this case, the 

 archaic species generally has its closest affinities with fossil rather than 

 with living members of its group. Now it is a general rule that such primi- 

 tive species resemble members of other groups more than do the more 

 specialized members of the same group. To paraphrase this, species which 

 are placed near the points of branching in the tree of life show especial 

 resemblances to other species in both branches. If each species were cre- 

 ated independently of all others, this fact would be inexphcable. Common 

 characters should then be distributed among various groups without re- 

 gard to their level of specialization. Yet, if the evolutionary theory be 

 correct, then the most primitive members of related groups are those 

 which have diverged the least from their common ancestry, and so primi- 

 tive species should illuminate the relationships between groups. This is 

 exactly what is found in nature. 



Another suggestive aspect of taxonomy is the difficulty of distinguishing 

 closely related species. In not a few cases, the ranges of variation of re- 

 lated species overlap. For example, in the genus Dendroica, the wood 

 warblers, it is possible to set up a series ranging from the yellow warbler 

 through magnolia, myrtle, and palm warblers to the chestnut-sided war- 

 bler in which the color pattern gradually shifts from predominantly yellow 

 to predominantly black. The gradations are so fine that expert knowledge 

 is needed to separate the species (Figure 6). Again, the fruit flies Dro- 

 sophila pseiidoohscura and D. persimiUs are so similar that statistical 

 analysis of populations is needed to distinguish them. Such situations 

 strongly suggest descent from rather recent common ancestors. 



EVIDENCE FROM COMPARATIVE ANATOMY 



As comparative anatomy is the field from which inferences of relationship 

 among animals are most commonly drawn, it is an especially important 

 source of evidence for evolution. If any particular organ system is studied 

 in diverse representatives of a single phylum, one gets the impression that 

 the system is based upon a prototype which is simply varied from class 

 to class ( with finer variations within each class ) . Examples from the com- 

 parative anatomy of the vertebrates will be discussed. 



The Vertebral CoEumn. The vertebral columns of all vertebrates origi- 

 nate from similar embryonic rudiments, four pairs of condensed mes- 

 enchymal masses in each somite, the arcualia, with associated looser 

 mesenchyme (Figure 7). From these simple beginnings, the typical ver- 

 tebra with its centrum, neural arch, spinous and transverse processes, and 

 its various articular processes, is formed. Among the Cyclostomata, the 

 most primitive of living vertebrates, only the two dorsal pairs of arcualia 

 ( Figure 8 ) are formed, and these do not advance much beyond the em- 

 bryonic condition, for they merely form two pairs of cartilage spikes 

 flanking the spinal cord. 



In the Chondrichthyes (sharks and their allies), a much more extensive 

 development occurs ( Figure 9 ) . The two dorsal pairs of arcualia ( = arch- 

 Si 



