Origin of Species Variorum

→ gone on accumulating for a very 1859 1860 1861

to go on continuously accumulating during a very 1866 1869

gone on continuously accumulating during a 1872

→ so that there may have been time sufficient 1861

in order to have given sufficient time 1859 1860

so that there may be time sufficient 1866 1869

sufficient 1872

→ will generally have to 1859 1860 1861

will have to 1866 1869

must 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

change must have lived in 1872

→ formation, fossiliferous throughout its thickness, 1861

fossiliferous formation 1859 1860

formation, 1866 1869 1872

→ can only be accumulated 1859 1860 1861

fossiliferous throughout its thickness, can accumulate only 1866

fossiliferous throughout its entire thickness, can accumulate only 1869 1872

→ that 1861 1866 1869 1872

in order to enable 1859 1860

→ species may 1861

species to 1859 1860

marine species may 1866 1869 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

more or less interrupted. 1872

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 accumulating for a very long period, →so that there may have been time sufficient for the slow process of

modification;

variation;

hence the deposit →will generally have to be a very thick one; and the species

under- going

undergoing

→modification will have had to live on the same

district

area

throughout

the

this

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

~~have~~

have

counterbalance

counterbalanced

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

~~often~~

often

tend to

submerge

sink

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

widely different

different

mineralogical composition, we may reasonably suspect that the process of deposition has been →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.