500 ESSENTIALS OF ZOOLOGY 



when set over against the fundamental similarities. Closely related 

 groups show numerous similarities in the form of homologies. The 

 flipper of the whale, the wing of the bat or the bird, the foreleg of 

 the cat and the arm of man all show the same general type of struc- 

 ture in spite of certain specific differences. There is seldom any 

 question of their phylogenetic relationship. 



The presence of series of similar parts on different segments of 

 the same animal and the various specializations of these parts show 

 a progressive development. The highly specialized walking legs, 

 uropod, claws, and feelers of crayfish, for example, have all devel- 

 oped from the simple swdmmeret type of appendage. They form a 

 serial homology and are also homologous to the appendages of all 

 other Crustacea a^ well. 



In higher forms of animals, such as man, there are numerous struc- 

 tures which seem to be useless and are even harmful in some in- 

 stances. These are spoken of as vestigial structures. Such parts 

 correspond in structure and plan to functional parts in other re- 

 lated forms, but are reduced morphologically and without the origi- 

 nal function. In man, one probably thinks first of the vermiform 

 appendix of the colon as such a structure. This is apparently func- 

 tionless in human beings and can be removed with no loss, but this 

 same organ in rabbits, some birds, and other animals is an extensive 

 and functional digestive organ. Man has a coccyx or vestigial tail, 

 and the frog has the urostyle. Pythons and porpoises, neither of 

 which has the least use for them, have vestigial hind limbs similar 

 to the functional ones of their relatives. The salivary glands of 

 certain snakes have become adapted as poison glands, certain sweat 

 glands have become milk glands, and gill arches have become sup- 

 porting structures of the tongue, larynx, and throat of adult ter- 

 restrial animals. The blood supply and nerves both follow the 

 phylogenetic changes of these organs. 



Most animal types seem not to have originated in their present 

 forms, but they seem to have undergone changes through the long 

 periods of geologic history. The explanation offered by modern 

 biologists for the anatomical relations and resemblances between 

 animals is that the individuals in any group have inherited a similar 

 plan of structure from the ancestors which were common to all mem- 

 bers of the group. In a group, such as the vertebrates, there have 

 been numerous modifications of various fundamental structures in 



