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lectures on evolution-第6章

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Here; however; we have to take into consideration that important

truth so well insisted upon by Lyell and by Darwinthe

imperfection of the geological record。 It can be demonstrated

that the geological record must be incomplete; that it can only

preserve remains found in certain favourable localities and

under particular conditions; that it must be destroyed by

processes of denudation; and obliterated by processes of

metamorphosis。 Beds of rock of any thickness crammed full of

organic remains; may yet; either by the percolation of water

through them; or by the influence of subterranean heat; lose all

trace of these remains; and present the appearance of beds of

rock formed under conditions in which living forms were absent。

Such metamorphic rocks occur in formations of all ages; and; in

various cases; there are very good grounds for the belief that

they have contained organic remains; and that those remains have

been absolutely obliterated。



I insist upon the defects of the geological record the more

because those who have not attended to these matters are apt to

say; 〃It is all very well; but; when you get into a difficulty

with your theory of evolution; you appeal to the incompleteness

and the imperfection of the geological record;〃 and I want to

make it perfectly clear to you that this imperfection is a great

fact; which must be taken into account in all our speculations;

or we shall constantly be going wrong。



You see the singular series of footmarks; drawn of its natural

size in the large diagram hanging up here (Fig。 2); which I owe

to the kindness of my friend Professor Marsh; with whom I had

the opportunity recently of visiting the precise locality in

Massachusetts in which these tracks occur。 I am; therefore; able

to give you my own testimony; if needed; that the diagram

accurately represents what we saw。 The valley of the Connecticut

is classical ground for the geologist。 It contains great beds of

sandstone; covering many square miles; which have evidently

formed a part of an ancient sea…shore; or; it may be; lake…

shore。 For a certain period of time after their deposition;

these beds have remained sufficiently soft to receive the

impressions of the feet of whatever animals walked over them;

and to preserve them afterwards; in exactly the same way as such

impressions are at this hour preserved on the shores of the Bay

of Fundy and elsewhere。 The diagram represents the track of some

gigantic animal; which walked on its hind legs。 You see the

series of marks made alternately by the right and by the left

foot; so that; from one impression to the other of the three…

toed foot on the same side; is one stride; and that stride; as

we measured it; is six feet nine inches。 I leave you; therefore;

to form an impression of the magnitude of the creature which; as

it walked along the ancient shore; made these impressions。





Fig。 2。Tracks of Brontozoum。





Of such impressions there are untold thousands upon these

sandstones。 Fifty or sixty different kinds have been discovered;

and they cover vast areas。 But; up to this present time; not a

bone; not a fragment; of any one of the animals which left these

great footmarks has been found; in fact; the only animal remains

which have been met with in all these deposits; from the time of

their discovery to the present daythough they have been

carefully hunted overis a fragmentary skeleton of one of the

smaller forms。 What has become of the bones of all these

animals? You see we are not dealing with little creatures; but

with animals that make a step of six feet nine inches; and their

remains must have been left somewhere。 The probability is; that

they have been dissolved away; and completely lost。



I have had occasion to work out the nature of fossil remains; of

which there was nothing left except casts of the bones; the

solid material of the skeleton having been dissolved out by

percolating water。 It was a chance; in this case; that the

sandstone happened to be of such a constitution as to set; and

to allow the bones to be afterward dissolved out; leaving

cavities of the exact shape of the bones。 Had that constitution

been other than what it was; the bones would have been

dissolved; the layers of sandstone would have fallen together

into one mass; and not the slightest indication that the animal

had existed would have been discoverable。



I know of no more striking evidence than these facts afford; of

the caution which should be used in drawing the conclusion; from

the absence of organic remains in a deposit; that animals or

plants did not exist at the time it was formed。 I believe that;

with a right understanding of the doctrine of evolution on the

one hand; and a just estimation of the importance of the

imperfection of the geological record on the other; all

difficulty is removed from the kind of evidence to which I have

adverted; and that we are justified in believing that all such

cases are examples of what I have designated negative or

indifferent evidencethat is to say; they in no way directly

advance the hypothesis of evolution; but they are not to be

regarded as obstacles in the way of our belief in that doctrine。



I now pass on to the consideration of those cases which; for

reasons which I will point out to you by and by; are not to be

regarded as demonstrative of the truth of evolution; but which

are such as must exist if evolution be true; and which therefore

are; upon the whole; evidence in favour of the doctrine。 If the

doctrine of evolution be true; it follows; that; however diverse

the different groups of animals and of plants may be; they must

all; at one time or other; have been connected by gradational

forms; so that; from the highest animals; whatever they may be;

down to the lowest speck of protoplasmic matter in which life

can be manifested; a series of gradations; leading from one end

of the series to the other; either exists or has existed。

Undoubtedly that is a necessary postulate of the doctrine of

evolution。 But when we look upon living Nature as it is; we find

a totally different state of things。 We find that animals and

plants fall into groups; the different members of which are

pretty closely allied together; but which are separated by

definite; larger or smaller; breaks; from other groups。 In other

words; no intermediate forms which bridge over these gaps or

intervals are; at present; to be met with。



To illustrate what I mean: Let me call your attention to those

vertebrate animals which are most familiar to you; such as

mammals; birds; and reptiles。 At the present day; these groups

of animals are perfectly well…defined from one another。 We know

of no animal now living which; in any sense; is intermediate

between the mammal and the bird; or between the bird and the

reptile; but; on the contrary; there are many very distinct

anatomical peculiarities; well…defined marks; by which the

mammal is separated from the bird; and the bird from the

reptile。 The distinctions are obvious and striking if you

compare the definitions of these great groups as they now exist。



The same may be said of many of the subordinate groups; or

orders; into which these great classes are divided。 At the

present time; for example; there are numerous forms of non…

ruminant pachyderms; or what we may call broadly; the pig tribe;

and many varieties of ruminants。 These latter have their

definite characteristics; and the former have their

distinguishing peculiarities。 But there is nothing that fills up

the gap between the ruminants and the pig tribe。 The two are

distinct。 Such also is the case in respect of the minor groups

of the class of reptiles。 The existing fauna shows us

crocodiles; lizards; snakes; and tortoises; but no connecting

link between the crocodile and lizard; nor between the lizard

and snake; nor between the snake and the crocodile; nor between

any two of these groups。 They are separated by absolute breaks。

If; then; it could be shown that this state of things had always

existed; the fact would be fatal to the doctrine of evolution。

If the intermediate gradations; which the doctrine of evolution

requires to have existed between these groups; are not to be

found anywhere in the records of the past history of the globe;

their absence is a strong and weighty negative argument against

evolution; while; on the other hand; if such intermediate forms

are to be found; that is so much to the good of evolution;

although; for reasons which I will lay before you by and by; we

must be cautious in our estimate of the evidential cogency of

facts of this kind。



It is a very remarkable circumstance that; from the commencement

of the serious study of fossil remains; in fact; from the time

when Cuvier began his brilliant researches upon those found in

the quarries of Montmartre; palaeontology has shown what she was

going to do in this matter; and what kind of evidence it lay in

her power to produce。



I said just now that; in the existing Fauna; the group of pig…

like animals and the group of ruminants are entirely distinct;

but one of the first of Cuvier's discoveries was an animal which

he called the Anoplotherium; and which proved to be; in a

great many important respects; intermediate in character between

the pigs; on the one hand; and the ruminants on the other。

Thus; research into the history of the past did; to a certain

extent; tend to fill up the breach between the group of

ruminants and the group of pigs。 Another remarkable animal

restored by the great French palaeontologist; the

Palaeotherium; similarly tended to connect together

animals to all appearance so different as the rhinoceros; the

horse; and the tapir。 Subsequent research has brought to light

multitudes of facts of the same order; and at the present day;

the investigations of such anatomists as Rutimeyer and Gaudry

have tended to fill up; more and more; the gaps in our existing

series of mammals; and to connect groups formerly thought to

be distinct。



But I think it may have an especial interest if; instead of

dealing with these examples; which would require a great deal of

tedious osteological detail; I take the case of birds and

reptiles; groups which; at the present day; are so clearly

distinguished from one another that there are perhaps no classes

of animals which; in popular apprehension; are more completely

separated。 Existing birds; as you are aware; are covered with

feathers; their anterior extremities; specially and peculiarly

modified; are converted into wings by the aid of which most of

them are able to fly; they walk upright upon two legs; and these

limbs; when they are considered anatomically; present a great

number of exceedingly remarkable peculiarities; to which I may

have occasion to advert incidentally as I go on; and which are

not met with; even a

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