GENERAL SUCCESSION OF ORGANIC TYPES. 53 



firmer basis than that which asserts that there has been a f 

 general succession of organic types, and that the appear- ' 

 ance of the lower forms of life has in the main preceded that 

 of the higher forms in point of time. In other words, it is 

 one of the generalisations of Palaeontology that there has 

 not only been a succession, but also a progression, of organic 

 types in proceeding from the earliest fossiliferous deposits 

 up to the present day. Whilst this general law remains, asi 

 we believe, unassailable, there are some important considera- 

 tions which must not be lost sight of. In the first place, it is 

 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 com- 

 pared 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 exist- 

 ence 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 generalisa- 

 tion, but it does not affect its value if fairly examined. The 

 lower sub-kingdoms of the Invertebrate 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 meta- 

 morphism to which they have been subjected, and they conse- 

 quently 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 simultane- 

 ously, 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 



