200 ORIGINAL ARTICLES. 



quite inapplicable to this group ; since even if the limits of such assem- 

 blages were extended so as to include what would elsewhere be accounted 

 genera, they would still be found so intimately connected by gradational 

 links, that definite lines of demarcation could not be drawn between 

 them. 



III. The only natural classification of the vast aggregate of diver- 

 sified forms which this group contains, will be one which ranges them 

 according to their mode and degree of divergence from a small number 

 of principal family types ; and any subordinate groupings of genera and 

 species which may be adopted for the convenience of description and 

 nomenclature, must be regarded merely as assemblages characterized by 

 the nature and degree of the modifications of the original type which 

 they may have respectively acquired in the course of genetic descent 

 from a common ancestry. 



IV. Even in regard to these family types, it may be fairly ques- 

 tioned whether analogical evidence does not rather favour the idea of 

 their derivation from a common original, than that of their primitive dis- 

 tinctness. 



V. The evidence in regard to the genetic continuity of the Forami- 

 nifera of successive geological periods and of those of the later of these 

 and the existing inhabitants of our seas, is as complete as the nature of 

 the case admits. 



VI. There is no evidence of any fundamental modification or advance 

 in the Foraminiferous type from the Palaeozoic period to the present time. 

 The most marked transition appears to have taken place between the 

 Cretaceous period, whose Foraminiferous Fauna seems to have been 

 chiefly composed of smaller and simpler types, and the commencement of 

 the Tertiary, of which one of the earliest members was the JNuminulitic 

 limestone, which forms a stratum of enormous thickness, that ranges over 

 wide areas in Europe, Asia, and America, and is chiefly composed of the 

 largest and most specialized forms of the entire group. But these were 

 not unrepresented in previous epochs ; and their extraordinary develop- 

 ment may have been simply due to the prevalence of conditions that 

 specially favoured it. The Foraminiferous Fauna of our own seas pro- 

 bably presents a greater range of variety than existed at any preceding 

 period ; but there is no indication of any tendency to elevation towards 

 a higher type. 



VII. The general principles thus educed from the study of the 

 Foraminifera should be followed in the investigation of the systematic 

 affinities of each of those great types of animal and vegetable form, 

 which is marked out by its physiological distinctness from the rest. 

 In every one of these there is ample evidence of variability ; and the 

 limits of that variability have to be determined by a far more extended 

 comparison than has been usually thought necessary, before the real 

 relations of their different forms can be even approximately deter- 

 mined. 



VIII. As it is the aim of the physical philosopher to determine 



