PREFACE. xi 



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 nomencla- 

 ture, must be regarded merely as assemblages of forms characterised 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 analogical 

 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 between the Foraminifera of succes- 

 sive geological periods, and between those of the later of these periods and the existing in- 

 habitants 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 transition 

 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 commence- 

 ment of the Tertiary series, of which one of the earliest members was the Nummulitic Lime- 

 stone, 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 epociis ; 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 eleva- 

 tion 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 tlie rest. 

 In every one of these there is ample evidence of variability ; and the limits of that varia- 

 bility have to be determined by a far more extended comparison than has been usually 

 thought necessary, before the real relations of their diS"erent forms can be even approximately 

 determined. 



