4i8 NATURAL SCIENCE. Dec, 



the careful researches which have been made of late years, no well- 

 recognised forms have been discovered in earlier rocks than the 

 Cambrian? 2 Certainly, only a few sediments have yet been dis- 

 covered of earlier date than the Cambrian which could have been 

 deposited under conditions suitable to the existence of animal life ; 

 but some have been found, and yet up to the present they have only 

 yielded very doubtful traces of organisms. The great physical 

 changes which affected a very large surface of the globe before the 

 Cambrian rocks were deposited possibly removed or obliterated much 

 important evidence ; but on the other hand it is more than probable 

 that sediments containing many earlier faunas than those at present 

 known to us are now submerged in the great oceanic basins. 



It is generally admitted by geologists that the ocean basins have 

 been great depressions of the earth's surface from a very early period 

 in the world's history, but this does not of course mean that the 

 same amount of land has been under water from that time to this. 

 We know, on the contrary, by geological evidence that, owing to 

 repeated physical changes, the area covered by water has varied 

 much in extent at different times from the Cambrian period to the 

 present ; and it seems but reasonable to suppose that it did so also 

 in Pre-Cambrian times. These changes, however, have been mainly 

 along the borders of the oceans ; hence the strata which have been 

 brought to the surface are all comparatively shallow-water deposits. 

 From what has been stated it would be anticipated that the earliest 

 known faunas would be likely to have a fairly wide distribution over 

 the globe, and that they would contain such forms only as could live 

 near shores or in comparatively shallow water and were not easily 

 affected by changes of temperature. 



If the earliest known faunas lived along the borders of the ocean 

 basins, it is but natural to suppose that pre-existing faunas, from 

 which they were descended, inhabited similar localities ; and we are 

 justified in supposing that these basins when first tenanted by 

 animal life were very much more limited in extent than they were at 

 the commencement of Cambrian time. The lowest Cambrian rocks, 

 being everywhere deposited near shore or in comparatively shallow 

 water, tell us clearly that the depression at that time was still going 

 on, and that the waters were spreading gradually over fresh land- 

 areas. Most of the evidence obtained of late years bearing on these 

 questions has been accumulated through researches in Europe and 

 North America ; therefore I propose to confine my further remarks 

 to those portions of the globe. 



As the surfaces of the Pre-Cambrian continents of Europe and 

 America were very irregular when the lowest Cambrian deposits 



- The paper by Professor W. K. Brooks, of which an account was given in the 

 October number of Natural Science (vol. v., p. 242), deals with this question in a 

 very interesting and nighly suggestive manner, and seems to me to explain some of 

 the difficulties which meet the palaeontologist in his inquiries. 



