640 



ORGANIC EVOLUTION 



teeth ) . Perhaps a few thousand generations 

 hence people may be born without wisdom 

 teeth, an appendix, or a tail. 



Vestigial structures in other animals often 

 tell us something about their life history. 

 The porpoise, for example, has a tiny pelvic 

 girdle (Fig. 25-15) buried deep in its hip 

 region where it could not possibly perform 



Fig. 25-15. Structures that performed a function at one 

 time in an animal's history often continue to appear 

 although today they are useless structures. Two good 

 examples are the posterior appendage bones in por- 

 poises (whales as well) and the remnants of similar 

 appendages remaining in the python. Porpoises have 

 no use for the bones, yet they always appear in every 

 animal. The observable external remnant of posterior 

 appendages in snakes occurs only in a few snakes 

 but indicates that the ancestors of present-day snakes 

 had legs. 



any function whatever, yet it is faithfully 

 produced in the body of every porpoise. 

 This vestige supports other evidence that 

 this animal was a four-footed land animal 

 at one time in the far distant past. Similarly, 

 the python possesses a iiidimentary pel- 

 vic girdle (Fig. 25-15), whereas all other 

 snakes are without appendages of any sort. 

 This vestige indicates that snakes had four- 

 footed ancestors in the dim, distant past. 



Anomalies in development occasionally 

 occur which give some hint as to the his- 

 tory of certain structures. They are caused 

 by the retention of embryonic structures in 

 the adult and are called atavisms or rever- 

 sions. For example, it is not uncommon for 

 a child to be bom with a well-developed 

 external tail, reminiscent of a similar struc- 



ture in the embryo (Fig. 25-16). Some- 

 times, additional nipples occur along lateral 

 lines that coincide with the milk ridge in 

 the embryo. In animals with multiple births 

 this arrangement of the mammary glands is 

 customary. Such "mistakes" in develop- 

 ment throw light on the history of the body. 



From comparative physiology and 

 biochemistry 



The very fact that all animals posses es- 

 sentially the same elements and chemical 

 compounds would indicate a common 

 origin. Even protoplasm is very similar 

 among all animals. The various enzymes 

 that are found in the digestive tract of man 

 can be identified in other animals, even 

 down to the Protozoa. Other intiacellular 

 and extracellular enzymes can likewise be 

 identified in all animals which have so far 

 been studied. It appears reasonable to 

 assume, then, that the chemical constitution 

 of two closely related species is more nearly 

 alike than that of two distantly related 

 species. Appropriate immunological and 

 biochemical tests of the various fluids and 

 tissues of a large variety of animals have 

 shown this to be the case. Blood is most 

 commonly studied because it is most 

 readily obtained and most easily handled in 

 the laboratory. By utilizing the antigen- 

 antibody technique described earlier (p. 

 510), it has been possible to verify the re- 

 lationships between many animals. It is 

 gratifying to note that the relationships 

 established by these blood tests corroborate 

 those erected by our system of classifica- 

 tion based on homologous stiTictures. For 

 example, it is possible to show that among 

 invertebrates Lwitilus is more closely re- 

 lated to the scorpions than any other 

 invertebrates, and among vertebrates that 

 seals and sea lions are more closely related 

 to one another than to other mammals. 

 Man's nearest kin among the primates has 

 been shown to be the great apes and his 

 most distant the lemur. The unifonnity of 

 birds has been verified by blood tests be- 



