28 THE AGE OF THE EARTH 



a double misapprehension, partly as to the facts, partly 

 as to the value of negative evidence, which may be 

 as good in its way as any other kind of evidence. 



Geologists are not unaware of the pitfalls which beset 

 negative evidence, and they do not conclude from the 

 absence of fossils in the rocks which underlie the 

 Cambrian that pre-Cambrian periods were devoid of 

 life ; on the contrary, they are fully persuaded that the 

 seas of those times were teeming with a rich variety 

 of invertebrate forms. How is it that, with the ex- 

 ception of some few species found in beds immediately 

 underlying the Cambrian, these have left behind no 

 vestige of their existence ? The explanation does not 

 lie in the nature of the sediments, which are not unfitted 

 for the preservation of fossils, nor in the composition of 

 the then existing sea water, which may have contained 

 quite as much calcium carbonate as occurs in our present 

 oceans ; and the only plausible supposition would appear 

 to be that the organisms of that time had not passed 

 beyond the stage now represented by the larvae of 

 existing invertebrata, and consequently were either un- 

 provided with skeletons, or at all events with skeletons 

 durable enough for preservation. If so, the history of 

 the earlier stages of the evolution of the invertebrata 

 will receive no light from palaeontology ; and no direct 

 answer can be expected to the question whether, eighteen 

 or nineteen millions of years being taken as sufficient for 

 the evolution of the vertebrata, the remaining available 

 eight millions would provide for that of the invertebrate 

 classes which are represented in the lowest Cambrian 

 deposits. On a priori grounds there would appear to 

 be no reason why it should not. If two millions of years 

 afforded time enough for the conversion of fish into 

 amphibians, a similar period should suffice for the 

 evolution of trilobites from annelids, or of annelids 



