142 THREE CRUISES OF THE “ BLAKE.” 
The 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-hundred-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 violent 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 — like 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- 
Па and cretaceous saurians, 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 transition 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 paleontological 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 limits 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 greater number of the marine forms characteristic 
