友情提示:如果本网页打开太慢或显示不完整,请尝试鼠标右键“刷新”本网页!阅读过程发现任何错误请告诉我们,谢谢!! 报告错误
九色书籍 返回本书目录 我的书架 我的书签 TXT全本下载 进入书吧 加入书签

lectures on evolution-第6章

按键盘上方向键 ← 或 → 可快速上下翻页,按键盘上的 Enter 键可回到本书目录页,按键盘上方向键 ↑ 可回到本页顶部!
————未阅读完?加入书签已便下次继续阅读!




and; indeed; it has been determined that; when these animals

lived; the falls of Niagara must have been at least six miles

further down the river than they are at present。

Many computations have been made of the rate at which the falls

are thus cutting their way back。 Those computations have varied

greatly; but I believe I am speaking within the bounds of

prudence; if I assume that the falls of Niagara have not

retreated at a greater pace than about a foot a year。 Six miles;

speaking roughly; are 30;000 feet; 30;000 feet; at a foot a

year; gives 30;000 years; and thus we are fairly justified in

concluding that no less a period than this has passed since the

shell…fish; whose remains are left in the beds to which I have

referred; were living creatures。



But there is still stronger evidence of the long duration of

certain types。 I have already stated that; as we work our way

through the great series of the Tertiary formations; we find

many species of animals identical with those which live at the

present day; diminishing in numbers; it is true; but still

existing; in a certain proportion; in the oldest of the Tertiary

rocks。 Furthermore; when we examine the rocks of the Cretaceous

epoch; we find the remains of some animals which the closest

scrutiny cannot show to be; in any important respect; different

from those which live at the present time。 That is the case with

one of the cretaceous lamp…shells (Terebratula); which

has continued to exist unchanged; or with insignificant

variations; down to the present day。 Such is the case with the

Globigerin?; the skeletons of which; aggregated together;

form a large proportion of our English chalk。

Those Globigerinae can be traced down to the

Globigerinae which live at the surface of the present

great oceans; and the remains of which; falling to the bottom of

the sea; give rise to a chalky mud。 Hence it must be admitted

that certain existing species of animals show no distinct sign

of modification; or transformation; in the course of a lapse of

time as great as that which carries us back to the Cretaceous

period; and which; whatever its absolute measure; is certainly

vastly greater than thirty thousand years。



There are groups of species so closely allied together; that it

needs the eye of a naturalist to distinguish them one from

another。 If we disregard the small differences which separate

these forms; and consider all the species of such groups as

modifications of one type; we shall find that; even among the

higher animals; some types have had a marvellous duration。

In the chalk; for example; there is found a fish belonging to

the highest and the most differentiated group of osseous fishes;

which goes by the name of Beryx。 The remains of that fish

are among the most beautiful and well…preserved of the fossils

found in our English chalk。 It can be studied anatomically; so

far as the hard parts are concerned; almost as well as if it

were a recent fish。 But the genus Beryx is represented;

at the present day; by very closely allied species which are

living in the Pacific and Atlantic Oceans。 We may go still

farther back。 I have already referred to the fact that the

Carboniferous formations; in Europe and in America; contain the

remains of scorpions in an admirable state of preservation; and

that those scorpions are hardly distinguishable from such as now

live。 I do not mean to say that they are not different; but

close scrutiny is needed in order to distinguish them from

modern scorpions。



More than this。 At the very bottom of the Silurian series; in

beds which are by some authorities referred to the Cambrian

formation; where the signs of life begin to fail useven there;

among the few and scanty animal remains which are discoverable;

we find species of molluscous animals which are so closely

allied to existing forms that; at one time; they were grouped

under the same generic name。 I refer to the well…known 

Lingula of the Lingula flags; lately; in

consequence of some slight differences; placed in the new genus

Lingulella。 Practically; it belongs to the same great

generic group as the Lingula; which is to be found at the

present day upon your own shores and those of many other parts

of the world。



The same truth is exemplified if we turn to certain great

periods of the earth's historyas; for example; the Mesozoic

epoch。 There are groups of reptiles; such as the

Ichthyosauria and the Plesiosauria; which appear

shortly after the commencement of this epoch; and they occur in

vast numbers。 They disappear with the chalk and; throughout the

whole of the great series of Mesozoic rocks; they present no

such modifications as can safely be considered evidence of

progressive modification。



Facts of this kind are undoubtedly fatal to any form of the

doctrine of evolution which postulates the supposition that

there is an intrinsic necessity; on the part of animal forms

which have once come into existence; to undergo continual

modification; and they are as distinctly opposed to any view

which involves the belief; that such modification may occur;

must take place; at the same rate; in all the different types of

animal or vegetable life。 The facts; as I have placed them

before you; obviously directly contradict any form of the

hypothesis of evolution which stands in need of these

two postulates。



But; one great service that has been rendered by Mr。 Darwin to

the doctrine of evolution in general is this: he has shown that

there are two chief factors in the process of evolution: one of

them is the tendency to vary; the existence of which in all

living forms may be proved by observation; the other is the

influence of surrounding conditions upon what I may call the

parent form and the variations which are thus evolved from it。

The cause of the production of variations is a matter not at all

properly understood at present。 Whether variation depends upon

some intricate machineryif I may use the phraseof the living

organism itself; or whether it arises through the influence of

conditions upon that form; is not certain; and the question may;

for the present; be left open。 But the important point is that;

granting the existence of the tendency to the production of

variations; then; whether the variations which are produced

shall survive and supplant the parent; or whether the parent

form shall survive and supplant the variations; is a matter

which depends entirely on those conditions which give rise to

the struggle for existence。 If the surrounding conditions are

such that the parent form is more competent to deal with them;

and flourish in them than the derived forms; then; in the

struggle for existence; the parent form will maintain itself and

the derived forms will be exterminated。 But if; on the contrary;

the conditions are such as to be more favourable to a derived

than to the parent form; the parent form will be extirpated and

the derived form will take its place。 In the first case; there

will be no progression; no change of structure; through any

imaginable series of ages; in the second place there will be

modification of change and form。



Thus the existence of these persistent types; as I have termed

them; is no real obstacle in the way of the theory of evolution。

Take the case of the scorpions to which I have just referred。

No doubt; since the Carboniferous epoch; conditions have always

obtained; such as existed when the scorpions of that epoch

flourished; conditions in which scorpions find themselves better

off; more competent to deal with the difficulties in their way;

than any variation from the scorpion type which they may have

produced; and; for that reason; the scorpion type has persisted;

and has not been supplanted by any other form。 And there is no

reason; in the nature of things; why; as long as this world

exists; if there be conditions more favourable to scorpions than

to any variation which may arise from them; these forms of life

should not persist。



Therefore; the stock objection to the hypothesis of evolution;

based on the long duration of certain animal and vegetable

types; is no objection at all。 The facts of this characterand

they are numerousbelong to that class of evidence which I have

called indifferent。 That is to say; they may afford no direct

support to the doctrine of evolution; but they are capable of

being interpreted in perfect consistency with it。



There is another order of facts belonging to the class of

negative or indifferent evidence。 The great group of Lizards;

which abound in the present world; extends through the whole

series of formations as far back as the Permian; or latest

Palaeozoic; epoch。 These Permian lizards differ astonishingly

little from the lizards which exist at the present day。

Comparing the amount of the differences bet
返回目录 上一页 下一页 回到顶部 0 0
未阅读完?加入书签已便下次继续阅读!
温馨提示: 温看小说的同时发表评论,说出自己的看法和其它小伙伴们分享也不错哦!发表书评还可以获得积分和经验奖励,认真写原创书评 被采纳为精评可以获得大量金币、积分和经验奖励哦!