Origin of Species Variorum

→ Cambrian and Silurian trilobites are 1872

Silurian trilobites have 1859 1860 1861 1866

Silurian trilobites are 1869

→ belonging to the same groups which have subsequently appeared, 1869 1872

of the orders to which they belong, 1859 1860 1861

belonging to the same group which have subsequently appeared, 1866

↑ 1 blocks not present in 1866 1869 1872; present in 1859 1860 1861

If, moreover, they had been the progenitors of these orders, they would almost certainly have been long ago supplanted and exterminated by their numerous and improved descendants.

→ stratum 1859 1860 1861 1866 1872

or Cambrian stratum 1869

→ periods 1869 1872

yet quite unknown, periods of time, 1859 1860 1861 1866

→ OMIT 1872

close of the 1869

→ rich fossiliferous deposits belonging to these assumed earliest periods prior to the Cambrian system, 1872

records of these vast primordial periods, 1859 1860 1861

richly fossiliferous records of these vast primordial periods, 1866

rich fossiliferous deposits belonging to these assumed earliest periods, 1869

→ OMIT 1866 1869 1872

of the most 1859 1860 1861

→ were until recently 1866 1869 1872

are 1859 1860 1861

suddenly appear in the lowest known fossiliferous rocks. Most of the arguments which have convinced me that all the existing species of the same group

have

are

descended from

one

a single

progenitor, apply with

nearly

....

equal force to the earliest known species. For instance,

cannot

doubt

be doubted

that all the →Cambrian and Silurian trilobites are descended from some one crustacean, which must have lived long before the

Silurian

Cambrian

age, and which probably differed greatly from any known animal. Some of the most ancient

Silurian

....

animals, as the Nautilus, Lingula,

&c,

&c.,

do not differ much from living species; and it cannot on

our

theory be supposed, that these old species were the progenitors of all the species →belonging to the same groups which have subsequently appeared, for they

are

not

present characters

....

in any degree intermediate

between them.

in character.

↑

Consequently, if

the

theory be true, it is indisputable

that,

that

before the lowest

Silurian

Cambrian

→stratum was

deposited

deposited,

long periods elapsed, as long as, or probably far longer than, the whole interval from the

Silurian

Cambrian

age to the present day; and that during these

vast,

vast

→periods the world swarmed with living creatures. Here we encounter a formidable objection; for it seems doubtful whether the

earth

earth,

in a fit state for the habitation of living

creatures

creatures,

has lasted long enough. Sir W. Thompson concludes that the consolidation of the crust can hardly have occurred less than 20 or more than 400 million years ago, but probably not less than 98 or more than 200 million years. These very wide limits show how doubtful the data are; and other elements may have

hereafter to

be introduced into the problem. Mr. Croll estimates that about 60 million years have elapsed since the Cambrian period, but this, judging from the small amount of organic change since the commencement of the Glacial epoch,

seems

appears

a very short time for the many and great mutations of life, which have certainly occurred since the Cambrian formation; and the previous 140 million years can hardly be considered as sufficient for the development of the varied forms of life which

certainly

already

existed

towards

during

the →OMIT Cambrian period. It is, however, probable, as Sir William Thompson insists, that the world at a very early period was subjected to more rapid and violent changes in its physical conditions than those now occurring; and such changes would have tended to induce changes at a corresponding rate in the organisms which then existed.

To the question why we do not find →rich fossiliferous deposits belonging to these assumed earliest periods prior to the Cambrian system, I can give no satisfactory answer. Several →OMIT eminent geologists, with Sir R. Murchison at their head, →were until recently convinced that we

see

beheld

in the organic remains of the