Origin of Species Variorum

→ gone on continuously accumulating during a 1872

gone on accumulating for a very 1859 1860 1861

to go on continuously accumulating during a very 1866 1869

→ sufficient 1872

in order to have given sufficient time 1859 1860

so that there may have been time sufficient 1861

so that there may be time sufficient 1866 1869

→ must 1872

will generally have to 1859 1860 1861

will have to 1866 1869

→ change must have lived in 1872

modification will have had to live on 1859 1860 1861

modification will have to live in 1866

change will have to live in 1869

→ formation, 1866 1869 1872

fossiliferous formation 1859 1860

formation, fossiliferous throughout its thickness, 1861

→ fossiliferous throughout its entire thickness, can accumulate only 1869 1872

can only be accumulated 1859 1860 1861

fossiliferous throughout its thickness, can accumulate only 1866

→ that 1861 1866 1869 1872

in order to enable 1859 1860

→ marine species may 1866 1869 1872

species to 1859 1860

species may 1861

→ more or less interrupted. 1872

much interrupted, as a change in the currents of the sea and a supply of sediment of a different nature will generally have been due to geographical changes requiring much time. 1859 1860 1861 1866 1869

→ length of time 1872

time 1859 1860 1861 1866 1869

→ which are elsewhere 1872

elsewhere 1859 1860 1861 1866 1869

these beds,

might

would

be tempted to conclude that the average duration of life of the embedded fossils had been less than that of the glacial period, instead of having been really far greater, that

is,

extending from before the glacial epoch to the present day.

In order to get a perfect gradation between two forms in the upper and lower parts of the same formation, the deposit

will

must

have →gone on continuously accumulating during a long period, →sufficient for the slow process of

variation;

modification;

hence the deposit →must be a very thick one; and the species

under- going

undergoing

→change must have lived in the same

area

district

throughout

this

the

whole time. But we have seen that a thick →formation, →fossiliferous throughout its entire thickness, can accumulate only during a period of subsidence; and to keep the depth approximately the same, which is necessary →that the same →marine species may live on the same space, the supply of sediment must nearly

have

....

counterbalanced

counterbalance

the amount of subsidence. But this same movement of subsidence will

often

....

tend to

sink

submerge

the area whence the sediment is derived, and thus diminish the

supply

supply,

whilst the downward movement continues. In fact, this nearly exact balancing between the supply of sediment and the amount of subsidence is probably a rare contingency; for it has been observed by more than one palæontologist, that very thick deposits are usually barren of organic remains, except near their upper or lower limits.

It would seem that each separate formation, like the whole pile of formations in any country, has generally been intermittent in its accumulation. When we see, as is so often the case, a formation composed of beds of

different

widely different

mineralogical composition, we may reasonably suspect that the process of deposition has been →more or less interrupted. Nor will the closest inspection of a formation give

any

us any

idea of the →length of time which its deposition

has

may have

consumed. Many instances could be given of beds only a few feet in thickness, representing formations, →which are elsewhere thousands of feet in thickness, and which must have required an enormous period for their accumulation; yet no one ignorant of this fact would have

suspected

even suspected

the vast lapse of time represented by the thinner formation. Many cases could be given of the lower beds of a formation having been upraised, denuded, submerged, and then re-covered by the upper beds of the same formation,— facts, showing what wide, yet easily overlooked, intervals have occurred in its accumulation. In other cases we have the plainest evidence in great fossilised trees, still standing upright as they grew, of many long intervals of time and changes of level during the process of deposition, which would

never even

not

have been