GRADUAL APPEARANCE AND DISAPPEARANCE OF SPECIES{326}
{326} This chapter corresponds to ch. X of _Origin_, Ed. i., vi.
ch. XI, "On the geological succession of organic beings."
In the Tertiary system, in the last uplifted beds, we find all the
species recent and living in the immediate vicinity; in rather older
beds we find only recent species, but some not living in the immediate
vicinity{327}; we then find beds with two or three or a few more extinct
or very rare species; then considerably more extinct species, but with
gaps in the regular increase; and finally we have beds with only two or
three or not one living species. Most geologists believe that the gaps
in the percentage, that is the sudden increments, in the number of the
extinct species in the stages of the Tertiary system are due to the
imperfection of the geological record. Hence we are led to believe that
the species in the Tertiary system have been gradually introduced; and
from analogy to carry on the same view to the Secondary formations. In
these latter, however, entire groups of species generally come in
abruptly; but this would naturally result, if, as argued in the
foregoing chapter, these Secondary deposits are separated by wide
epochs. Moreover it is important to observe that, with our increase of
knowledge, the gaps between the older formations become fewer and
smaller; geologists of a few years standing remember how beautifully
has the Devonian system{328} come in between the Carboniferous and
Silurian formations. I need hardly observe that the slow and gradual
appearance of new forms follows from our theory, for to form a new
species, an old one must not only be plastic in its organization,
becoming so probably from changes in the conditions of its existence,
but a place in the natural economy of the district must [be made,] come
to exist, for the selection of some new modification of its structure,
better fitted to the surrounding conditions than are the other
individuals of the same or other species{329}.
{327} _Origin_, Ed. i. p. 312, vi. p. 453.
{328} In the margin the author has written "Lonsdale." This refers
to W. Lonsdale's paper "Notes on the age of the Limestone of South
Devonshire," _Geolog. Soc. Trans._, Series 2, vol. V. 1840, p. 721.
According to Mr H. B. Woodward (_History of the Geological Society
of London_, 1907, p. 107) "Lonsdale's 'important and original
suggestion of the existence of an intermediary type of Palaeozoic
fossils, since called Devonian,' led to a change which was then
'the greatest ever made at one time in the classification of our
English formations'." Mr Woodward's quotations are from Murchison
and Buckland.
{329} <Note in original.> Better begin with this. If species really,
after catastrophes, created in showers over world, my theory false.
<In the above passage the author is obviously close to his theory of
divergence.>
In the Tertiary system the same facts, which make us admit as probable
that new species have slowly appeared, lead to the admission that old
ones have slowly disappeared, not several together, but one after
another; and by analogy one is induced to extend this belief to the
Secondary and Palaeozoic epochs. In some cases, as the subsidence of a
flat country, or the breaking or the joining of an isthmus, and the
sudden inroad of many new and destructive species, extinction might be
locally sudden. The view entertained by many geologists, that each fauna
of each Secondary epoch has been suddenly destroyed over the whole
world, so that no succession could be left for the production of new
forms, is subversive of my theory, but I see no grounds whatever to
admit such a view. On the contrary, the law, which has been made out,
with reference to distinct epochs, by independent observers, namely,
that the wider the geographical range of a species the longer is its
duration in time, seems entirely opposed to any universal
extermination{330}. The fact of species of mammiferous animals and fish
being renewed at a quicker rate than mollusca, though both aquatic; and
of these the terrestrial genera being renewed quicker than the marine;
and the marine mollusca being again renewed quicker than the Infusorial
animalcula, all seem to show that the extinction and renewal of species
does not depend on general catastrophes, but on the particular relations
of the several classes to the conditions to which they are exposed{331}.
{330} Opposite to this passage the author has written "d'Archiac,
Forbes, Lyell."
{331} This passage, for which the author gives as authorities the
names of Lyell, Forbes and Ehrenberg, corresponds in part to the
discussion beginning on p. 313 of _Origin_, Ed. i., vi. p. 454.
Some authors seem to consider the fact of a few species having
survived{332} amidst a number of extinct forms (as is the case with a
tortoise and a crocodile out of the vast number of extinct sub-Himalayan
fossils) as strongly opposed to the view of species being mutable. No
doubt this would be the case, if it were presupposed with Lamarck that
there was some inherent tendency to change and development in all
species, for which supposition I see no evidence. As we see some species
at present adapted to a wide range of conditions, so we may suppose that
such species would survive unchanged and unexterminated for a long time;
time generally being from geological causes a correlative of changing
conditions. How at present one species becomes adapted to a wide range,
and another species to a restricted range of conditions, is of difficult
explanation.
{332} The author gives Falconer as his authority: see _Origin_, Ed.
i. p. 313, vi. p. 454.
_Extinction of species._
The extinction of the larger quadrupeds, of which we imagine we better
know the conditions of existence, has been thought little less wonderful
than the appearance of new species; and has, I think, chiefly led to the
belief of universal catastrophes. When considering the wonderful
disappearance within a late period, whilst recent shells were living, of
the numerous great and small mammifers of S. America, one is strongly
induced to join with the catastrophists. I believe, however, that very
erroneous views are held on this subject. As far as is historically
known, the disappearance of species from any one country has been
slow--the species becoming rarer and rarer, locally extinct, and finally
lost{333}. It may be objected that this has been effected by man's
direct agency, or by his indirect agency in altering the state of the
country; in this latter case, however, it would be difficult to draw any
just distinction between his agency and natural agencies. But we now
know in the later Tertiary deposits, that shells become rarer and rarer
in the successive beds, and finally disappear: it has happened, also,
that shells common in a fossil state, and thought to have been extinct,
have been found to be still living species, but very _rare_ ones{334}.
If the rule is that organisms become extinct by becoming rarer and
rarer, we ought not to view their extinction, even in the case of the
larger quadrupeds, as anything wonderful and out of the common course of
events. For no naturalist thinks it wonderful that one species of a
genus should be rare and another abundant, notwithstanding he be quite
incapable of explaining the causes of the comparative rareness{335}. Why
is one species of willow-wren or hawk or woodpecker common in England,
and another extremely rare: why at the Cape of Good Hope is one species
of rhinoceros or antelope far more abundant than other species? Why
again is the same species much more abundant in one district of a
country than in another district? No doubt there are in each case good
causes: but they are unknown and unperceived by us. May we not then
safely infer that as certain causes are acting _unperceived_ around us,
and are making one species to be common and another exceedingly rare,
that they might equally well cause the final extinction of some species
without being perceived by us? We should always bear in mind that there
is a recurrent struggle for life in every organism, and that in every
country a destroying agency is always counteracting the geometrical
tendency to increase in every species; and yet without our being able to
tell with certainty at what period of life, or at what period of the
year, the destruction falls the heaviest. Ought we then to expect to
trace the steps by which this destroying power, always at work and
scarcely perceived by us, becomes increased, and yet if it continues to
increase ever so slowly (without the fertility of the species in
question be likewise increased) the average number of the individuals of
that species must decrease, and become finally lost. I may give a single
instance of a check causing local extermination which might long have
escaped discovery{336}; the horse, though swarming in a wild state in La
Plata, and likewise under apparently the most unfavourable conditions in
the scorched and alternately flooded plains of Caraccas, will not in a
wild state extend beyond a certain degree of latitude into the
intermediate country of Paraguay; this is owing to a certain fly
depositing its eggs on the navels of the foals: as, however, man with a
_little_ care can rear horses in a tame state _abundantly_ in Paraguay,
the problem of its extinction is probably complicated by the greater
exposure of the wild horse to occasional famine from the droughts, to
the attacks of the jaguar and other such evils. In the Falkland Islands
the check to the _increase_ of the wild horse is said to be loss of the
sucking foals{337}, from the stallions compelling the mares to travel
across bogs and rocks in search of food: if the pasture on these islands
decreased a little, the horse, perhaps, would cease to exist in a wild
state, not from the absolute want of food, but from the impatience of
the stallions urging the mares to travel whilst the foals were too
young.
{333} This corresponds approximately to _Origin_, Ed. i. p. 317,
vi. p. 458.
{334} The case of _Trigonia_, a great Secondary genus of shells
surviving in a single species in the Australian seas, is given as
an example in the _Origin_, Ed. i. p. 321, vi. p. 463.
{335} This point, on which the author laid much stress, is
discussed in the _Origin_, Ed. i. p. 319, vi. p. 461.
{336} _Origin_, Ed. i. p. 72, vi. p. 89.
{337} This case does not occur in the _Origin_, Ed.
From our more intimate acquaintance with domestic animals, we cannot
conceive their extinction without some glaring agency; we forget that
they would undoubtedly in a state of nature (where other animals are
ready to fill up their place) be acted on in some part of their lives by
a destroying agency, keeping their numbers on an average constant. If
the common ox was known only as a wild S. African species, we should
feel no surprise at hearing that it was a very rare species; and this
rarity would be a stage towards its extinction. Even in man, so
infinitely better known than any other inhabitant of this world, how
impossible it has been found, without statistical calculations, to judge
of the proportions of births and deaths, of the duration of life, and of
the increase and decrease of population; and still less of the causes of
such changes: and yet, as has so often been repeated, decrease in
numbers or rarity seems to be the high-road to extinction. To marvel at
the extermination of a species appears to me to be the same thing as to
know that illness is the road to death,--to look at illness as an
ordinary event, nevertheless to conclude, when the sick man dies, that
his death has been caused by some unknown and violent agency{338}.
{338} An almost identical sentence occurs in the _Origin_, Ed. i.
p. 320, vi. p. 462.
In a future part of this work we shall show that, as a general rule,
groups of allied species{339} gradually appear and disappear, one after
the other, on the face of the earth, like the individuals of the same
species: and we shall then endeavour to show the probable cause of this
remarkable fact.
{339} _Origin_, Ed. i. p. 316, vi. p. 457.