PALEONTOLOGY 855 



times. With this great span of time (150,000,000 years)* one can 

 partly understand how each species of mammal today has one or 

 more species of tapeworms adapted to life in the body of its host 

 only. 



Some pathogenic bacteria when attacking an animal body leave 

 unmistakable lesions in various parts of the skeleton. The late 

 Dr. Roy L. Moodie, working first at the Baylor Medical School and 

 later at the University of Illinois, was able to date the appearance 

 on the earth of many important disease bacteria. Dr. Moodie 

 studied large collections of skeletons of ancient reptiles and mam- 

 mals and found lesions which the modem pathologist associates 

 with the various bacteria in question. 



Before leaving the matter of indirect records, one more instance 

 will be mentioned. Some predatory carnivorous dinosaurs left on 

 the vertebrae and other bones of their victims, deep tooth prints 

 and scoriations as the only record which keeps them from oblivion. 

 Are these tooth scratches fossils! 



The last fifteen years have seen an unprecedented rise in the in- 

 terest in paleontology and in the number of full-time workers in 

 this field. Shortly after World War I, the pressing search for 

 more petroleum brought out strongly the value of fossils in estab- 

 lishing "horizons*' or levels which the oil geologist could use in 

 determining favorable domes and other structures into which to 

 drill. Immediately, workers in what had, hitherto, been one of the 

 most academic subjects were lured from classrooms and museum 

 benches into one of the most competitive and commercial industries. 



At first attention was given only to the usual larger fossils found 

 at the surface, but as the science of petroleum geology advanced, 

 it became more and more important to know the levels or horizons 



*How we have come to change such expressions of the earlier geologists as "an- 

 cient," "very ancient" and "extremely ancient" to round figures is a long and fasci- 

 nating story which cannot be taken up here. The students of geochronology or age 

 of the earth after many unsuccessful attempts based on rate of increase of salinity 

 of the sea, change of the ellipticity of the earth's orbit, addition of the elements of 

 the known geological column and others which produced figures which made us 

 human beings, earth's most recent inhabitants, gasp, have now found a dependable 

 check on absolute time. 



The earlier estimates, and that is all they were, are characterized by only one fea- 

 ture common to all and that is a large figure for the age of the earth. As large as 

 these figures were, they have been replaced by others which are truly staggering, and 

 the new figures are believed accurate. At least different workers get closely parallel 

 results. 



The unvarying time clock of a disintegrating radio-active mineral and its use in 

 determining the absolute length of time since the beginning of the various geologic 

 periods are discussed fully in any new book on historical geology or perhaps more 

 entertainingly in Wells' Science of Life or Jeans' Through Space and Time 



