Darwin, and After Darwin Volume I Part 19

3. Similarly, and more generally, the Dicotyledonous plants, which first appear in the Cretaceous rocks, appear there suddenly, without any forms leading up to them--notwithstanding that "we know very well the extensive flora of the underlying Wealden." Moreover, we have all the three great divisions of the Dicotyledons appearing together, and so highly differentiated that all the species are referred to existing genera, with the exception of a very few imperfectly preserved, and therefore uncertain fragments.

Such being the facts, we may begin by noticing that, even at first sight, they present different degrees of difficulty. Thus, I cannot see that there is much difficulty with regard to those in cla.s.s 2. Only if we were to take the popular (and very erroneous) view of organic evolution as a process which is always and everywhere bound to promote the specialization of organic types--only then ought we to see any real difficulty in the absence of generalized types preceding these existing types. Of course we may wonder why still lower down in the geological series we do not meet with more generalized (or ancestral) types; but this is the difficulty number 3, which we now proceed to examine.

Concerning the other two difficulties, then, the only possible way of meeting that as to the absence of any parent forms lower down in the geological series is by falling back--as in the a.n.a.logous case of animals--upon the imperfection of the geological record. Although it is certainly remarkable that we should not encounter any forms serving to connect the Dicotyledonous plants of the Chalk with the lower forms of the underlying Wealden, we must again remember that difficulties thus depending on the absence of any corroborative record, are by no means equivalent to what would have arisen in the presence of an adverse record--such, for instance, as would have been exhibited had the floras of the Wealden and the Chalk been inverted. But, as the case actually stands, the mere fact that Dicotyledonous plants, where they first occur, are found to have been already differentiated into their three main divisions, is in itself sufficient evidence, on the general theory of evolution, that there must be a break in the record as. .h.i.therto known between the Wealden and the Chalk. Nor is it easy to see how the opponents of this theory can prove their negative by furnis.h.i.+ng evidence to the contrary. And although such might justly be deemed an unfair way of putting the matter, were this the only case where the geological record is in evidence, it is not so when we remember that there are numberless other cases where the geological record does testify to connecting links in a most satisfactory manner. For in view of this consideration the burden of proof is thrown upon those who point to particular cases where there is thus a conspicuous absence of transitional forms--the burden, namely, of proving that such cases are not due merely to a break in the record. Besides, the break in the record as regards this particular case may be apparent rather than real.

For I suppose there is no greater authority on the pure geology of the subject than Sir Charles Lyell, and this is what he says of the particular case in question. "If the pa.s.sage seem at present to be somewhat sudden from the flora of the Lower or Neocomian to that of the Upper Cretaceous period, the abruptness of the change will probably disappear when we are better acquainted with the fossil vegetation of the uppermost tracts of the Neocomian and that of the lowest strata of the Gault, or true Cretaceous series[59]."

[59] _Elements of Geology_, p. 280.

Lastly, the fact of the flora of the glacial epoch not having exhibited any modifications during the long residence of some of its specific types in Great Britain and elsewhere, is a fact of some importance to the general theory of evolution, since it shows a higher degree of stability on the part of these specific types than might perhaps have been expected, supposing the theory to be true. But I do not see that this const.i.tutes a difficulty against the theory, when we have so many other cases of proved trans.m.u.tation to set against it. For instance, not to go further afield than this very glacial flora itself, it will be remembered that in an earlier chapter I selected it as furnis.h.i.+ng specially cogent proof of the trans.m.u.tation of species. What, then, is the explanation of so extraordinary a difference between Mr. Carruthers'

views and my own upon this point? I believe the explanation to be that he does not take a sufficiently wide survey of the facts.

To begin with, it seems to me that he exaggerates the vicissitudes to which the species of plants that he calls into evidence have been exposed while advancing before, and retreating after, the ice. Rather do I agree with Darwin that "they would not have been exposed during their long migrations to any great diversity of temperature; and as they all migrated in a body together, their mutual relations will not have been much disturbed; hence, in accordance with the principles indicated in this volume, these forms will not have been liable to much modification[60]." But, be this matter of opinion as it may, a much better test is afforded by those numerous cases all the world over, where arctic species have been left stranded on alpine areas by the retreat of glaciation; because here there is no room for differences of opinion as to a "change of environment" having taken place. Not to speak of climatic differences between arctic and alpine stations, consider merely the changes which must have taken place in the relations of the thus isolated species to each other, as well as to those of all the foreign plants, insects, &c., with which they have long been thrown into close a.s.sociation. If in _such_ cases no variation or trans.m.u.tation had taken place since the glacial epoch, then indeed there would have been a difficulty of some magnitude. But, by parity of reasoning, whatever degree of difficulty would have been thus presented is not merely discharged, but converted into at least an equal degree of corroboration, when it is found that under such circ.u.mstances, in whatever part of the world they have occurred, some considerable amount of variation and trans.m.u.tation has always taken place,--and this in the animals as well as in the plants. For instance, again to quote Darwin, "If we compare the present Alpine plants and animals of the several great European mountain-ranges one with another, though many of the species remain identically the same, some exist as varieties, some as doubtful forms or sub-species, and some as distinct yet closely allied species representing each other on the several ranges[61]." Lastly, if instead of considering the case of alpine floras, we take the much larger case of the Old and New World as a whole, we meet with much larger proofs of the same general facts. For, "during the slowly decreasing warmth of the Pliocene period, as soon as the species in common, which inhabited the New and Old Worlds, migrated south of the Polar Circle, they will have been completely cut off from each other.

This separation, as far as the more temperate productions are concerned, must have taken place long ages ago. As the plants and animals migrated southward, they will have become mingled in one great region with the native American productions, and would have had to compete with them; and, in the other great region, with those of the Old World.

Consequently we have here everything favourable for much modification,--for far more modification than with the Alpine productions left isolated, within a much more recent period, on the several mountain ranges and on the arctic lands of Europe and N.

America. Hence it has come, that when we compare the now living productions of the temperate regions of the New and Old Worlds, we find very few identical species; but we find in every cla.s.s many forms, which some naturalists rank as geographical races, and others as distinct species; and a host of closely allied or representative forms which are ranked by all naturalists as specifically distinct[62]."

[60] _Origin of Species_, p. 332.

[61] _Origin of Species_, p. 332.

[62] _Ibid_. pp. 333-4.

In view then of all the above considerations--and especially those quoted from Darwin--it appears to me that far from raising any difficulty against the theory of evolution, the facts adduced by Mr.

Carruthers make in favour of it. For when once these facts are taken in connection with the others above mentioned, they serve to complete the correspondence between degrees of modification with degrees of time on the one hand, and with degrees of evolution, of change of environment, &c., on the other. Or, in the words of Le Conte, when dealing with this very subject, "It is impossible to conceive a more beautiful ill.u.s.tration of the principles we have been trying to enforce[63]."

[63] _Evolution and its Relation to Religious Thought_, p. 194.

NOTE A TO PAGE 257.

The pa.s.sages in Dr. Whewell's writings, to which allusion is here made, are somewhat too long to be quoted in the text. But as I think they deserved to be given, I will here reprint a letter which I wrote to _Nature_ in March, 1888.

In his essay on the _Reception of the Origin of Species_, Prof.

Huxley writes:--

"It is interesting to observe that the possibility of a fifth alternative, in addition to the four he has stated, has not dawned upon Dr. Whewell's mind" (_Life and Lectures of Charles Darwin_, vol. ii, p. 195).

And again, in the article _Science_, supplied to _The Reign of Queen Victoria_, he says:--

"Whewell had not the slightest suspicion of Darwin's main theorem, even as a logical possibility" (p 365).

Now, although it is true that no indication of such a logical possibility is to be met with in the _History of the Inductive Sciences_, there are several pa.s.sages in the _Bridgewater Treatise_ which show a glimmering idea of such a possibility. Of these the following are, perhaps, worth quoting. Speaking of the adaptation of the period of flowering to the length of a year, he says:--

"Now such an adjustment must surely be accepted as a proof of design, exercised in the formation of the world. Why should the solar year be so long and no longer? or, this being such a length, why should the vegetable cycle be exactly of the same length? Can this be chance?... And, if not by chance, how otherwise could such a coincidence occur than by an intentional adjustment of these two things to one another; by a selection of such an organization in plants as would fit them to the earth on which they were to grow; by an adaptation of construction to conditions; of the scale of construction to the scale of conditions? It cannot be accepted as an explanation of this fact in the economy of plants, that it is necessary to their existence; that no plants could possibly have subsisted, and come down to us, except those which were thus suited to their place on the earth. This is true; but it does not at all remove the necessity of recurring to design as the origin of the construction by which the existence and continuance of plants is made possible. A watch could not go unless there were the most exact adjustment in the forms and positions of its wheels; yet no one would accept it as an explanation of the origin of such forms and positions that the watch would not go if these were other than they were. If the objector were to suppose that plants were originally fitted to years of various lengths, and that such only have survived to the present time as had a cycle of a length equal to our present year, or one which could be accommodated to it, we should reply that the a.s.sumption is too gratuitous and extravagant to require much consideration."

Again, with regard to "the diurnal period," he adds:--

"Any supposition that the astronomical cycle has occasioned the physiological one, that the structure of plants has been brought to be what it is by the action of external causes, or that such plants as could not accommodate themselves to the existing day have perished, would be not only an arbitrary and baseless a.s.sumption, but, moreover, useless for the purposes of explanation which it professes, as we have noticed of a similar supposition with respect to the annual cycle."

Of course these pa.s.sages in no way make against Mr. Huxley's allusions to Dr. Whewell's writings in proof that, until the publication of the _Origin of Species_, the "main theorem" of this work had not dawned on any other mind, save that of Mr. Wallace.

But these pa.s.sages show, even more emphatically than total silence with regard to the principle of survival could have done, the real distance which at that time separated the minds of thinking men from all that was wrapped up in this principle. For they show that Dr. Whewell, even after he had obtained a glimpse of the principle "as a logical possibility," only saw in it an "arbitrary and baseless a.s.sumption." Moreover, the pa.s.sages show a remarkable juxtaposition of the very terms in which the theory of natural selection was afterwards formulated. Indeed, if we strike out the one word "intentional" (which conveys the preconceived idea of the writer, and thus prevented him from doing justice to any naturalistic view), all the following parts of the above quotations might be supposed to have been written by a Darwinian. "If not by chance, how otherwise could such a coincidence occur, than by an _adjustment_ of these two things to one another; by a _selection_ of such an organization in plants as would _fit_ them to the earth on which they were to grow; by an adaptation of _construction_ to _conditions_; of the _scale_ of construction to the _scale_ of conditions?" Yet he immediately goes on to say: "If the objector were to suppose that plants were originally _fitted_ to years of various lengths, and that such only have _survived_ to the present time ... _as could be accommodated to it_ (i. e. the actual cycle), we should reply that the a.s.sumption is too gratuitous and extravagant to require much consideration." Was there ever a more curious exhibition of failure to perceive the importance of a "logical possibility"? And this at the very time when another mind was bestowing twenty years of labour on its "consideration."

NOTE B TO PAGE 295.

Since these remarks were delivered in my lectures as here printed, Mr.

Mivart has alluded to the subject in the following and precisely opposite sense:--

Many of the more noteworthy instincts lead us from manifestations of purpose directed to the maintenance of the individual, to no less plain manifestations of a purpose directed to the preservation of the race. But a careful study of the interrelations and interdependencies which exist between the various orders of creatures inhabiting this planet shows us yet a more noteworthy teleology--the existence of whole orders of such creatures being directed to the service of other orders in various degrees of subordination and augmentation respectively. This study reveals to us, as a fact, the enchainment of all the various orders of creatures in a hierarchy of activities, in harmony with what we might expect to find in a world the outcome of a First Cause possessed of intelligence and will[64].

[64] _On Truth_, p. 493.

Having read this much, a Darwinian is naturally led to expect that Mr.

Mivart is about to offer some examples of instincts or structures exemplifying what in the margin he calls the "Hierarchy of Ministrations." Yet the only facts he proceeds to adduce are the sufficiently obvious facts, that the inorganic world existed before the organic, plants before herbivorous animals, these before carnivorous, and so on: that is to say, everywhere the conditions to the occurrence of any given stage of evolution preceded such occurrence, as it is obvious that they must, if, as of course it is not denied, the possibility of such occurrence depended on the precedence of such conditions. Now, it is surely obvious that such a "hierarchy of ministrations" as this, far from telling against the theory of natural selection, is the very thing which tells most in its favour. The fact that animals, for instance, only appeared upon the earth after there were plants for them to feed upon, is clearly a necessity of the case, whether or not there was any design in the matter. Such "ministrations,"

therefore, as plant-organisms yield to animal-organisms is just the kind of ministration that the theory of natural selection requires. Thus far, then, both the theories--natural selection and super-natural design--have an equal right to appropriate the facts. But now, if in no one instance can it be shown that the ministration of plant-life to animal-life is of such a kind as to subserve the interests of animal-life without at the same time subserving those of the plant-life itself, then the fact makes wholly in favour of the naturalistic explanation of such ministration as appears. If any plants had presented any characters pointing prospectively to needs of animals without primarily ministering to their own, then, indeed, there would have been no room for the theory of natural selection. But as this can nowhere be alleged, the theory of natural selection finds all the facts to be exactly as it requires them to be: such ministration as plants yield to animals becomes so much evidence of natural selection having slowly formed the animals to appropriate the nutrition which the plants had previously gathered--and gathered under the previous influence of natural selection acting on themselves entirely for their own sakes.

Therefore I say it is painfully manifest that "the enchainment of all the various orders of creatures in a hierarchy of activities," is _not_ "in harmony with what we might expect to find in a world the outcome of a First Cause possessed of intelligence and [beneficent] will." So far as any argument from such "enchainment" reaches, it makes entirely against the view which Mr. Mivart is advocating. In point of fact, there is a total absence of any such "ministration" by one "order of creatures" to the needs of any other order, as the beneficent design theory would necessarily expect; while such ministration as actually does obtain is exactly and universally the kind which the naturalistic theory requires.

Again, quite independently, and still more recently, Mr. Mivart alluded in _Nature_ (vol. xli, p. 41) to the difficulty which the apparently exceptional case of gall-formation presents to the theory of natural selection. Therefore I supplied (vol. xli, p. 80) the suggestion given in the text, viz. that although it appears impossible that the sometimes remarkably elaborate and adaptive structures of galls can be due to natural selection acting directly on the plants themselves--seeing that the adaptation has reference to the needs of their parasites--it is quite possible that the phenomena may be due to natural selection acting indirectly on the plants, by always preserving those individual insects (and larvae) the character of whose secretions is such as will best induce the particular shapes of galls that are required. Several other correspondents took part in the discussion, and most of them accepted the above explanation. Mr. T. D. A. c.o.c.kerell, however, advanced another and very ingenious hypothesis, showing that there is certainly one conceivable way in which natural selection might have produced all the phenomena of gall-formation by acting directly on the plants themselves[65]. Subsequently Mr. c.o.c.kerell published another paper upon the subject, stating his views at greater length. The following is the substance of his theory as there presented:--

[65] _Nature_, vol. xli, p. 344.

Doubtless there were internal plant-feeding larvae before there were galls: and, indeed, we have geological evidence that boring insects date very far back indeed. The primitive internal feeders, then, were miners in the roots, stems, twigs, or leaves, such as occur very commonly at the present day. These miners are excessively harmful to plant-life, and form a cla.s.s of the most destructive insect-pests known to the farmer: they frequently cause the death of the whole or part of the plant attacked. Now, we may suppose that the secretions of certain of these insects caused a swelling to appear where the larvae lived, and on this excrescence the larvae fed. It is easy to see that the greater the excrescence, and the greater the tendency of the larvae to feed upon it, instead of destroying the vital tissues, the smaller is the amount of harm to the plant. Now the continued life and vitality of the plant is beneficial to the larvae, and the larger or more perfect the gall, the greater the amount of available food. Hence natural selection will have preserved and acc.u.mulated the gall-forming tendencies, as not only beneficial to the larvae, but as a means whereby the larvae can feed with least harm to the plant. So far from being developed for the exclusive benefit of the larvae, it is easy to see that, allowing a tendency to gall-formation, natural selection would have developed galls exclusively for the benefit of the plants, so that they might suffer a minimum of harm from the unavoidable attacks of insects.

But here it may be questioned--have we proof that internal feeders tend to form galls? In answer to this I would point out that gall-formation is a peculiar feature, and cannot be expected to arise in every group of internal feeders. But I think we can afford sufficient proof that wherever it has arisen it has been preserved; and further, that even the highly complex forms of galls are evolved from forms so simple that we hesitate to call them galls at all[66].

[66] _Entomologist_, March, 1890.

The paper then proceeds to give a number of individual cases. No doubt the princ.i.p.al objection to which Mr. c.o.c.kerell's hypothesis is open is one that was pointed out by Herr Wetterhan, viz. "the much greater facility afforded to the indirect action through insects, by the enormously more rapid succession of generations with the latter than with many of their vegetable hosts--oaks above all[67]." This difficulty, however, Mr. c.o.c.kerell believes maybe surmounted by the consideration that a growing plant need not be regarded as a single individual, but rather as an a.s.semblage of such[68].

[67] _Nature_, vol. xli, p. 394.

[68] _Ibid._ vol. xli, pp. 559-560.

NOTE C TO PAGE 394.

The only remarks that Mr. Wallace has to offer on the _pattern of colours_, as distinguished from a mere _brilliancy of colour_, are added as an afterthought suggested to him by the late Mr. Alfred Tylor's book on _Colouration of Animals and Plants_ (1886). But, in the first place, it appears to me that Mr. Wallace has formed an altogether extravagant estimate of the value of this work. For the object of the work is to show, "that diversified colouration follows the chief lines of structure, and changes at points, such as the joints, where function changes." Now, in publis.h.i.+ng this generalization, Mr. Tylor--who was not a naturalist--took only a very limited view of the facts. When applied to the animal kingdom as a whole, the theory is worthless; and even within the limits of mammals, birds, and insects--which are the cla.s.ses to which Mr. Tylor mainly applies it--there are vastly more facts to negative than to support it. This may be at once made apparent by the following brief quotation from Prof. Lloyd Morgan:--

It can hardly be maintained that the theory affords us any adequate explanation of the _specific_ colour-tints of the humming-birds, or the pheasants, or the Papilionidae among b.u.t.terflies. If, as Mr.