14:2 THREE CRUISES OF THE " BLAKE."" 



Tlie littoral fauna of to-day occupies a somewhat narrow belt 

 within the hundred-fathom line, at no very great distance from 

 the shore ; in fact, inside of what has been termed the continen- 

 tal line (one-hundi-ed-fathom line), while the rest of the bottom 

 of the ocean is occupied by the continental and the strictly 

 deep-sea fauna. 



Littoral deposits take place under constantly changing condi- 

 tions, and are subject to \dolent perturbations, which disturb 

 and modify the stratification. Deep-sea formations, on the 

 contrary, must of necessity be subject to less interference, and 

 therefore acquire a greater thickness. If this was really the 

 case with the older formations, those which — Hke some of the 

 limestones — are of deep-sea origin, will be found to be much 

 thicker than the contemporaneous littoral deposits. This we 

 may assert to be the case. Care must of course be taken, as 

 has been urged by Fuchs, to compare such deposits as are 

 comparable, and not to draw deductions from phenomena which 

 have nothing in common. A coral fauna, a brachiopod fauna, 

 and an ammonite fauna give us no terms of comparison. We 

 may even have marked contrasts, as between eocene mamma- 

 lia and cretaceous saurian s, while a common factor is presented 

 by the flora of the two periods. Invertebrates pass from the 

 jura to the chalk without presenting striking contrasts, while 

 among fishes the ti-ansition from ganoids to teleosteans shows 

 the relationship in one case to be with the past, in the other 

 with the future. They are the young and the old generations. 

 The effect of surroundings is of course very different upon 

 organisms of various classes at successive periods of their geo- 

 logical and palseontological development. The attempt, made 

 by Fuchs, to identify the deep-sea formations of former geo- 

 logical periods from the facies of the fauna is subject to the 

 same difficulties that are found in determining the upper and 

 lower Hmits of 'the abyssal fauna. These difficulties are due 

 to the enormous bathymetrical range of many species, which 

 were at first regarded as strictly abyssal, but have subsequently 

 been proved to extend almost into the confines of the littoral 

 zone. 



By far the gTeater number of the marine forms characteristic 



