GENERAL SUCCESSION OF ORGANIC TYPES. 91 



very doubtful if we are as yet acquainted with the absolute 

 time of the first appearance upon the globe of even one of 

 the sub-kingdoms. Future discoveries, therefore, are almost 

 certain to push back still further into the remote vistas of 

 the past the point of time at which each morphological type 

 first made its appearance upon the globe. Still, there is 

 little likelihood that the relative times of appearance of the 

 great groups, as compared with one another, will be affected 

 by the researches of the future. It remains almost certain 

 that we shall find that the lower types were followed in 

 point of time by the higher. 



In the second place, we find all the primary types in 

 existence before the close of the Silurian period; and he 

 would be rash indeed who would dogmatically deny that 

 they might all have been present in the earlier Cambrian 

 period. This, at first sight, might seem almost to negative 

 the above generalisation, but it does not affect its value if 

 fairly examined. The lower sub-kingdoms of #ie Inverte- 

 brate animals appeared so early that their origin is lost in 

 the mists of antiquity, and we can say nothing positively as 

 to the time when each came into existence. The Cambrian 

 deposits are underlaid by the vast series of the Laurentian 

 deposits, representing an incalculable lapse of time. These 

 ancient sediments, with one exception, have hitherto proved 

 barren of life, owing to the intense metamorphism to which 

 they have been subjected, and they consequently yield no 

 evidence bearing on the question in hand. They serve to 

 show us, however, by their presence alone, that we must 

 in the meanwhile leave the Invertebrate sub-kingdoms out 

 of account altogether as bearing upon the question of the 

 succession and progression of organic types. We do not know 

 when these sub-kingdoms commenced, and hence we have 

 no right to assert either that they were all introduced simul- 

 taneously, or that they came into being successively. We 

 may be sure, however, of one thing they did not commence 

 at the points where now we find their earliest traces. There 

 remains, then, only the sub-kingdom of the Vertebrate animals 

 which can reasonably be appealed to as evidence on this 

 question. The stratified series is long enough to render it 



