64 GEOLOGICAL BIOLOGY. 



as satisfactory in dealing with the time-scale to discard them 

 altogether, and to consider the divisions as units which, added 

 together, make up the total duration of time from the foot of 

 the scale to the top, or to present time. 



Adopting this plan, each of the periods in the time-scale 

 on page 54 may be considered as a unit of time of uncertain 

 length, but of definite position in the scale; and the several 

 periods may, until evidence is found for a closer estimate, be 

 considered to be of equal value. This makes the time-ratios 

 to approach nearly the estimate made by Walcott, dividing 

 the whole scale from the base of the Cambrian into 20 

 such units and assigning 13 of them to the Paleozoic, 4 to 

 the Mesozoic, 3 to the Cenozoic time. Walcott's values were 

 19 units, and 12, 5, and 2 for the Paleozoic, Mesozoic, and 

 Cenozoic times respectively. In the scale here adopted there 

 is one probable exaggerated error, i.e., the more recent 

 units were probably relatively shorter than the more ancient 

 units which are represented of equal length. 



The time-scale as provisionally adopted is as follows : Di- 

 viding the total time represented by the faunas and floras 

 from the earliest Cambrian to the present time into one hun- 

 dred units, there are found to be twenty distinguishable and 

 pretty universally recognized biological life-periods, which 

 for convenience may be assumed to represent equal periods of 

 time, each period representing one twentieth or five per cent 

 of the whole. There are three of these periods in the Cam- 

 brian era, two in the Ordovician, etc. ; therefore the eras rep- 

 resent in percentages : the Cambrian, 15^; Ordovician, 10^; 

 Silurian, lO^; Devonian, 15^; Carboniferous, 15^; Triassic, 

 5^; Jurassic, 5^; Cretaceous, 10^; Tertiary, 10^; Quater- 

 nary and Recent, 5^. Paleozoic time is thus 65^^, Mesozoic 

 20<fc, Cenozoic 15^ of the whole. 



These estimates, for the purpose of measuring the rela- 

 tive duration of organic forms and thus the progress of the 

 history of organisms, have a rough approximation to the truth 

 according to the cumulative evidence from all sides at present 

 before us, but they must be accepted as provisional estimates 

 to be perfected by evidence which will come with the prog- 

 ress of knowledefe. 



