158 ELEMENTS OF BIOLOGY. 



less have remained unbroken. From this point of view 

 there would be little impropriety in saying that we are still 

 living in the Silurian period ; but we could only say so in a 

 very limited sense. Most geologists would freely admit 

 that there must in nature have been an actual continuity of 

 the great geological periods. Nevertheless it remains cer- 

 tain that we can never dispense with the division of the 

 stratified series into definite rock-groups and life-periods. 

 We can never hope to discover all the lost links of the 

 geological chain ; and the great formations are likely ever to 

 remain separated by more or less pronounced physical or 

 palasontological breaks, or both combined. The utmost we 

 can at present do is to arrive at the conviction that the 

 lines of demarcation between the great formations only 

 mark gaps in our knowledge, and that there can be truly no 

 hiatus in the long series of fossiliferous deposits. 



Imperfection of the Pal^ontological Record. — 

 As has been 'just pointed out, the series of the stratified 

 formations is to be regarded as an imperfect one, in which 

 many links are missing. The causes of this " imperfection 

 of the geological record," as it has been termed by Darwin, 

 are various ; but the most important ones are our as yet 

 limited knowledge of vast areas of the earth's surface, the 

 process of denudation, and the fact that many of the miss- 

 ing groups are buried beneath other deposits, whilst more 

 than half of the superficies of the globe is hidden from us 

 by the waters of the sea. The imperfection of the geological 

 record necessarily implies an equal imperfection of the 

 " palseontological record ; " but, in truth, the record of life 

 is far more imperfect than the mere physical series of de- 

 posits. The following are the chief causes of the imperfec- 

 tion of the palaeontological record : — 



I. In the first place, even if the series of the stratified 

 deposits had been preserved to us in its entirety, and we could 

 point to sedimentary accumulations belonging to every 

 period in the earth's history, there would still have been 



