STRUCTURE OF THE VERTEBRATES 335 



shown in all their relationships; and even striated muscle has 

 been found. The last was described in a fossil shark (Cladose- 

 lache) by Dean. This could hardly be petrifaction for the animal 

 was buried in mud, and the discovery evidences the marked dif- 

 ference in the chemical composition between the light and dark 

 bands of the tissue. Such illustrations could be multiplied many 

 times, despite the fact that palaeontological histology has hardly 

 been touched. 



In conclusion it may be said that the anatomy of many fossils 

 is almost as well known as though the animals were living. The 

 attachment of muscles leaves a roughness on the bone, and in 

 this way the musculature can be "dissected" with great ac- 

 curacy. Casts of the calvaria show the gross anatomy of the 

 brain, even to the convolutions; and with these facts in hand 

 the activity and the habits of the animal are more than specu- 

 lation, although one has to be guarded in conclusions regarding 

 the analogy between living animals and the dead. 



B. Geological Time Table 



The known sedimentary rocks have been divided into great 

 eras. Each of these divisions, which covered enormous lengths 

 of time, is divided into periods, and each of these into minor 

 subdivisions. Thus, by locating the stratum in which it occurs, 

 the relative age of the fossil is known. There is a remarkable 

 similarity in the grade of life found in the same period, no 

 matter where on the earth's surface it is found. There are ex- 

 ceptions to this rule, as illustrated by the distribution of mam- 

 mals in Australia, but in such cases other animals and the 

 sequence of strata make identification accurate. 



The scientist is not as much concerned over the number of 

 years involved as he is in the position of the strata, but many 

 attempts have been made to give approximate dates. The 

 biologists made the effort to determine the number of years 

 which would be required for certain evolutionary processes to 

 take place, but recent genetic research has shown that rays 

 which are present in light will greatly increase the rate of muta- 

 tions in the germplasm. Stratigraphers, by studying seasonable 

 depositions of minute strata, gave an idea of the time required 



