A Three-fold Development. 155 



Laurentian age. All rocks below the Cambrian were 

 only a few years ago called the Azoic, a word Avhich 

 means, without life. Later inA^estigations by geologists 

 have shown convincing proof that these rocks also were 

 laid down in an ocean teeming with life. Beginning with 

 the Cambrian age we find these rocks, in all parts of the 

 world where they have been examined, filled with fos- 

 sils. For many years this fact was a stumbling block 

 in the way of the theory of evolution which it was dif- 

 ficult for its adherents to explain aw^ay. The sudden 

 beginning of such a multitude of life forms in these 

 rocks, while, apparently, there was no life in the forma- 

 tions lying just below, was a puzzle for a long time. But 

 the patient study of scores of students has shown that 

 the seeming absence of life was caused by the metamor- 

 phism of the Laurentian rocks, which has destroyed all 

 direct traces of fossils. The Laurentian rocks are im- 

 mensely greater in age than the Cambrian strata, which 

 rest uncomformably upon them. It is believed that the 

 time since life began was very much longer before the 

 Cambrian period than has passed from that time to the 

 present. In other words, the lifetime on the ear-th has 

 been more than doubled by these later discoveries. Dana 

 says the Cambrian rocks have furnished no fossils of- 

 terrestrial life, all plants and animals were marine. Other 

 authors have claimed a very few uncertain forms of land 

 plants. The plants, with the possible exception noted 

 above, were all of the kind called fucoids, while the 

 animal forms were all invertebrates. There would be 

 nothing strange in the absence of land ]3lants among 

 these fossils, even if they then existed, as these are much 

 less likely to be fossilized and ijreserved than sea plants. 

 The land plant nearly always grows, dies and decays 

 where its seed first germinates. There would be but few 

 which would fall into the water and be preserved as fos- 

 sils, while a large part of marine plants would be covered 

 up with sediment, and so iDreserved from decay. We 

 must, therefore, leave the question yet undecided as to 

 whether there was any land vegetation during the Cam- 

 brian period. During the whole of the Silurian age, of 

 which the Cambrian was the lower member, there has 

 as yet been found no forms of land life, if we except a 



