THii: ORIGIN AND HISTORY OF ANIMAL LIFE 



615 



seals and bears are more closely related to 

 each other than they are to dogs. 



Additional evidence of animal relation- 

 ships is provided by the degree of similarity 

 between the blood protein of various ani- 

 mals. Thousands of tests by the antigen- 

 antibody technique such as is used for blood 

 grouping have shown a fundamental like- 

 ness between the blood proteins of all mam- 

 mals. These blood tests have shown that 

 man is closest genetically to the great ape; 

 next closest, in order, are the Old World 

 monkeys, the New World monkeys, and 

 the tarsioids. Blood protein tests given to 

 other animals give proof that dogs, cats, and 

 bears are closely related, while sheep, goats, 

 cows, antelopes, and deer form another 

 closely related group. Sea lions and seals are 

 more closely related to the carnivores than 

 to the other mammals, as anatomic facts 

 also indicate. 



Hormones show similar reactions in dif- 

 ferent animals. If beef or sheep thyroid is 

 fed to frog tadpoles, from which the thyroid 

 gland has been removed, they metamorphose 

 into normal frogs. The hormones used in 

 treating endocrine deficiencies in man are 

 obtained from other vertebrates. 



Another point of similarity in most ani- 

 mals is the presence of enzymes essentially 

 alike in their physiologic action. For exam- 

 ple, trypsin, which acts upon proteins, oc- 

 curs in many animals from protozoans to 

 man; and amylase, which acts on starches, is 

 present from sponges to man. It seems rea- 

 sonable to suppose that similarity of bio- 

 chemistry exists in such widely different 

 animals because they have developed from 

 a common ancestor. If each animal were a 

 separate and special creation, then it is dif- 

 ficult to understand why such similarities in 

 chemistry should run through living things. 



Geographic distribution 



According to students of geographic dis- 

 tribution, each species arose in a definite 

 area, which is known as its center of origin; 



from this area it tended to disperse, as al- 

 ready described. Many facts of present-day 

 distribution can only be explained on the 

 assumption that organic evolution has taken 

 place. For example, continental islands pos- 

 sess faunas most closely related to those of 

 the continents near which they are located, 

 whereas the faunas of oceanic islands appear 

 to have arisen by chance introduction of 

 species from various regions. Isolation evi- 

 dently plays an important role in evolution. 

 Thus many animals on continental islands 

 have evidently changed sufficiently, since 

 these islands were cut off from the 

 mainland, to be recognized as distinct 

 species. 



Fossil animals: historic 

 record of evolution 



From Darwin's time to the present, the 

 remains of ancient animal life (fossils) have 

 been regarded as strong evidence of evolu- 

 tion. 



Fossil remains of animals are usually petri- 

 factions, that is, parts that have been re- 

 placed by mineral matter, but the hard 

 parts of the animals, such as bones, shells, 

 and teeth, may be preserved intact, and 

 parts of or entire animals may be preserved 

 in ice (frozen mammoths in Siberia), am- 

 ber (insects), asphalt (saber-tooth cats), 

 and oil-bearing soil (mammoths). Casts of 

 animals resulting from the dissolution of the 

 hard parts and the filling up of the space 

 with mineral matter, also give some idea of 

 external features. Which animals become 

 fossilized and which of these are discovered 

 are more or less matters of chance. Further- 

 more, fossils are usually broken by the forces 

 of nature and are therefore fragmentary 

 when they are obtained. Nevertheless, from 

 the evidence obtained from these fossils, 

 paleontologists have constructed the accom- 

 panying table showing the geologic periods, 

 arranged in the order of their succession, and 

 the approximate time of origin of the dif- 

 ferent groups of animals (p. 617). 



