CLASS/F/C.l T/O.VS TN NATURAL HISTORY. 209 



methods agree in the general results; and while there is a 

 vast amount to learn, to which future theories must adjust, 

 the general facts in the case, which alone we are considering 

 in these lectures, are already fairly well established. 



Mature Individuals, not Embryos, used by the Paleontologist, — 

 The chief difference between the two points of view, as they 

 concern us, is that the paleontological method deals essen- 

 tially with the matured results of individual development. 

 It is remains of the mature organisms that he investigates, 

 and he examines the differences between the mature individ- 

 uals of the successiv^e periods ; while in the other method it 

 is the rudimentary conditions of individuals that carry the 

 evidence of the affinity. 



Differentiation attained during the First or Cambrian Era. — 

 The paleontologist asks, To what extent has differentiation 

 proceeded in the individuals of any particular geological 

 ■epoch, and on comparing the fossils of successive epochs, in 

 what respects and at what rate has differentiation proceeded? 

 In carrying out this method of study we inquire, first. To 

 what extent has morphological differentiation reached in the 

 ■first geological age of which we have record, i.e., the Cam- 

 brian? In reply the answer may be briefly given in terms 

 of abstract scientific nomenclature, by stating the numerical 

 relation existing between the number of the branches and of 

 the classes of the Animal Kingdom which are known to have 

 lived in Cambrian time and the total known number in each 

 category. 



On page 206 is given a table of the branches and classes of 

 the Animal Kingdom of which record is preserved in the rocks, 

 Avith their known geological range. In this summary we 

 may omit from consideration the branches Tunicata and Ver- 

 tebrata, of which we have no evidence in Cambrian time; and 

 the Protozoa may be omitted from the consideration because, 

 although it is altogether probable that they were well repre- 

 sented, traces of them are almost entirely wanting on account 

 of the minuteness and simplicity of their forms. We may 

 also omit the consideration of such classes as the Holothuri- 

 oidea, of which no evidence is found in a fossil state. And, 

 finally, taking all the other branches, classes, and orders, 



