554 - Heredity and Evolution 



several invertebrate phyla (Nemertinea, 

 Rotifera, etc., p. 680). All in all, therefore, 

 classification, or taxonomy, is a very difficult 

 science, fraught with uncertainty and con- 

 troversy. But experience has shown that this 

 natural system is very useful. In giving the 

 phylum, or class, or other lesser category of 

 any animal or plant, the name of the group 

 immediately specifies that this organism 

 possesses a long list of structural, functional, 

 and developmental characteristics, in com- 

 mon with all other members of the same 

 group. 



COMPARATIVE ANATOMY IN RELATION 

 TO EVOLUTION 



Each large group of organisms, such as the 

 phyla and classes of the animal and plant 

 kingdoms, displays a fundamental architec- 

 tural plan of body structure that underlies 

 the diverse specializations of the different 

 body parts. Invariably all the members of 

 such a group possess a number of homolo- 

 gous organs that display unmistakable simi- 

 larities of structure and development, al- 

 though the particular organ in the different 

 species may be adapted to totally different 

 functions. Generally speaking, the differences 

 between homologous organs show an adapta- 

 tive relationship to differences in the en- 

 vironments of the several species — as would 

 be expected as a result of natural selection. 

 But the similarities between homologous 

 organs seem to have no logical explanation, 

 except in terms of genetic relationship. 



To exemplify homology, let us take the 

 skeletal system of vertebrate animals, or more 

 particularly the bony structure of the fore- 

 limbs throughout the vertebrate group. Re- 

 gardless of whether the forelimb is adapted 

 for walking, swimming, flying, or other spe- 

 cial function, the same grouping of homolo- 

 gous bones can easily be recognized in any 

 vertebrate — as is shown in Figure 29-1. This 

 does not mean that such a skeletal structure 

 is ideally suited for all these purposes. The 

 walking, swimming, and flying appendages 



of insects are just as well adapted to each of 

 these same functions, although insect ap- 

 pendages display a totally different plan of 

 structure. The differences between the fore- 

 limbs of various vertebrates are obviously 

 correlated with their different environ- 

 ments and functions, while their underlying 

 similarities — as to the form and arrangement 

 of the homologous bones and muscles — must 

 be ascribed to the fact that all vertebrates 

 are genetically related to each other more 

 closely than to other animals. 



Similarly the whole skeletal system, as well 

 as all the muscles, nerves, sensory organs, 

 digestive organs, reproductive structures, etc., 

 of vertebrate animals display innumerable 

 homologies of structure and development. 

 The same general architectural plan can be 

 seen in the body structure of every verte- 

 brate, and no other group of animals con- 

 forms to this particular plan. Variation and 

 selection have modified the plan to fit a wide 

 variety of special environmental conditions 

 in a wide variety of species; but the plan 

 itself remains to testify as to the common 

 inheritance that all vertebrates have re- 

 ceived from a common ancestral stock. 



Vestigial Organs. Some organs in every 

 species appear to be quite useless and de- 

 generate as compared to homologous struc- 

 tures in related species, and such vestigial 

 organs are by no means rare. The complete 

 list of vestigial structures in the human body, 

 for example, probably numbers more than a 

 hundred; and the list includes a number of 

 familiar structures, such as the vermiform 

 appendix, the coccyx (tail vertebrae), the 

 nictitating membrane of the eye, and the 

 small muscles that can be trained to "move 

 the ears." Moreover, comparable numbers of 

 vestigial organs are found in other groups 

 of animals and plants — in proportion to the 

 complexity of their structure, and to the care 

 and thoroughness with which they have been 

 studied. 



A vestigial organ is presumed to represent 

 the surviving remnant of an organ or struc- 

 ture that was well developed and functional 



