THE GEOLOGICAL RECORD 289 



used as an argument against the theory of organic evolution. 

 From the nature of the case it must be imperfect and the 

 surprising thing about it is that it should have yielded so much 

 evidence as it has done. The earlier portions of the record, how- 

 ever, have apparently been completely destroyed. The fact that 

 already in the Cambrian epoch highly organized invertebrate 

 animals, such as trilobites (Fig. 134) and brachiopods, were in 

 existence, shows us that evolution must then have been going on 

 for an immensely long period. This period, as we have already 

 indicated, is represented by the pre-Cambrian rocks, in which 

 the organic remains have been so far destroyed as to be only 

 recognizable in a few doubtful cases. 



The investigation of what remains of the geological record 

 has, however, yielded some very remarkable and conclusive 

 results. In the first place we have learnt that life on the earth 

 has been continuous without a break from the commencement 

 of the Cambrian epoch to the present day, and in the second 

 place we have learnt that the higher groups of animals and 

 plants have appeared on the earth in exactly the order which we 

 should expect on the assumption that each has arisen from some 

 preceding and more lowly organized ancestral group. 



In discussing this point we may confine our attention to the 

 Vertebrata, the evolution of which seerns to have taken place 

 entirely during post-Cambrian times. The most lowly organized 

 group of true vertebrates existing at the present day is that of 

 the Cyclostomata, which includes the lampreys and hag-fishes. 

 These are distinguished from true fishes by the absence of jaws, 

 the mouth being suctorial, and by other primitive features. The 

 skull and vertebral column remain cartilaginous throughout life 

 and there are no paired limbs. Neither is any dermal armour 

 of any kind developed. The only structures which they possess 

 which seem at all likely to be preserved as fossils are the horny 

 teeth. Certain bodies known as conodonts, which occur as 

 fossils in strata ranging from the Lower Silurian to the 

 Carboniferous, may perhaps represent such teeth. If so they 

 are perhaps the earliest vertebrate remains as yet known. 1 



The remarkable group of fishes known as Ostracodermi, 

 however, makes its appearance in the Upper Silurian and 

 continues on to the Upper Devonian. These animals agree 



1 Palaeospondylus, from the Lower Old Red Sandstone of Scotland, is probably 

 a l*o a fyclcstorne. 



