THE EVOLUTION OF ORGANIC TYPES IN TIME. 97 



portant points in connection with this subject may, however, be 

 shortly considered under the following heads : — 



1. The Primordial Types of Life. — As above remarked, we know 

 nothing, and are never likely to know anything, of the animals and 

 plants which really constituted the first living beings. Of the life of 

 the Archaean period we at present have no certain knowledge ; but 

 we find representatives of all the Invertebrate sub-kingdoms in the 

 earliest fossiliferous deposits (Cambrian and Ordovician), while Ver- 

 tebrates appear low down in the Silurian (in the Clinton formation 

 of North America). It may, however, be taken as certain that these 

 ancient fossils cannot possibly be the remains of the really primor- 

 dial forms of life. Thus, regarded as individuals, these old organic 

 types are as complex and as highly specialised in their structure as 

 are the animals now in existence. Moreover, the great Invertebrate 

 groups of the Annulosa and Mollusca are found in the Cambrian 

 period to be represented by many diverse forms, and to have already 

 reached a stage of advanced development. It would, however, be 

 at variance with all that we learn from the study of existing organisms, 

 that these great morphological types should, to begin with, have 

 presented themselves under highly specialised forms. Rather must 

 we conclude that a very long period must have been required for the 

 evolution of these varied morphological types, and that the Cambrian 

 fauna was really preceded by many antecedent faunae which are at 

 present unknown to us. 



2. The Introduction of 'New Species. — From the beginning of the 

 Cambrian period onwards, new species of animals have been intro- 

 duced upon the earth, apparently almost continuously. We may 

 certainly say that the introduction of new species has been " con- 

 tinuous," if we use this term in the sense " of the continued opera- 

 tion of the cause or causes which introduced life at first " (Dawson). 

 It has long been recognised that at certain periods in geological his- 

 tory large numbers of new species were introduced, and this was 

 formerly explained on the supposition that life was periodically de- 

 stroyed by physical convulsions, and that each of these " catas- 

 trophes " was followed by a creation of new animals and plants. 

 The apparent periodicity in the introduction of new species is, how- 

 ever, probably really due simply to the imperfection of the geological 

 record. In all those cases, therefore, where we meet with the appar- 

 ently sudden incoming of a large number of new life-forms, we may 

 take it for granted that we have to deal with a hiatus in the geolo- 

 gical record in the particular area in which this phenomenon is ob- 

 served. The new forms have, namely, been in existence elsewhere, 

 and what we are observing is not their first introduction upon the 

 earth, but merely their first introduction into the area in question. 

 As regards the period in which we are now living, any apparent ces- 



vol. 1. G 



