GENEEAL CONCLUSIONS. 585 



of such assemblages 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 diversified forms which 

 this group contains, will be one which ranges them according to their direction 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 of forms 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 fairly be questioned whether analo- 

 gical evidence does not rather favour the idea of their derivation from a common original, 

 than that of their primitive distinctness. 



V. The evidence in regard to the genetic continuity of the Foraminifera of successive 

 geological periods, and of those of the later of these periods 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 Forami- 

 niferous type from the Palaeozoic period to the present time. The most marked trans- 

 ition appears to have taken place between the Cretaceous period, whose Foraminiferous 

 fauna seems to have been chiefly composed of the smaller and simpler types, and the 

 commencement of the Tertiary series, of which one of the earliest members was the 

 Nummulitic 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 pre- 

 vious epochs ; and their extraordinary development may have been simply due to the 

 prevalence of conditions that specially favoured it. The Foraminiferous fauna of our 

 own seas probably 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 approximatively determined. 



VIII. As it is the aim of the Physical Philosopher to determine " what are the fewest 

 and simplest assumptions, which being granted, the whole existing order of nature would 

 result*," so the aim of the philosophic Naturalist should be to determine how small a 

 number of primitive types may be reasonably supposed to have given origin by the ordi- 

 nary course of " descent with modification " to the vast multitude of diversified forms 



* MILL'S Logic, 3rd edition, vol. i. p. 327. 

 HDCCCLX. 4 II 



