Weismann August
Studies in the Theory of Descent, Volume I
VI. Objections to a Phyletic Vital Force
It has been shown in the previous section that the three elements composing the markings of the Sphinx-larvæ originally possessed a distinct significance with respect to the life of the species, and that they were by this means called into existence. It has likewise been shown, that in most of the species which possess these characters at the present time they still have a decided, although sometimes a different use, for their possessors, so that from this point of view no objection can be raised to their being considered as having arisen by natural selection.
On looking at the phenomenon as a whole, however, certain instances occur which appear quite irreconcilable with this view.
The most formidable objection is offered by the genus Deilephila. The row of ring-spots which nearly all the existing species have more or less developed, has arisen from a simple subdorsal line. It would not, therefore, be surprising if a species were discovered which possessed this line without any ring-spots as its only marking. If D. Hippophaës were thus marked, there would be no objection to the theoretical assumption that this155 was the ancestor of the other species. It would then be said that ring-spots were first developed in a later species by natural selection, and that they had been transmitted to all succeeding and younger species.
Certain individuals of D. Hippophaës, however, possess small ring-spots, some of which are well developed on several segments. In this species the row of ring-spots is therefore comprised in the development. The remaining species, which are much younger phyletically than Hippophaës, could not have inherited their ring-spots from the latter, since this species itself only possesses them occasionally, and, so to speak, in a tentative manner. The spots would therefore appear to have arisen spontaneously in this species, and independently of those in the other species. But if this were the case, how should we be able to prove that in the other species also the ring-spots did not arise independently; and if, moreover, a large number of species showed the same character without its being referable to inheritance from a common ancestor, how could this be otherwise explained than as the result of a force innate in these species and producing similar variations? But this is nothing but Askenasy’s “fixed direction of variation” —i. e., a phyletic vital force.
The only escape from this difficulty is perhaps to be found in proving that D. Hippophaës formerly possessed ring-spots, and that these have been subsequently either partially or completely lost, so that their occasional appearance in this species would therefore depend upon reversion. The ontogeny, however, teaches us that this is not the case, since the young caterpillar does not possess a greater number of more distinct ring-spots, but wants them altogether with the exception of a red spot on the eleventh segment, which is, however, much fainter than in the last stage.
This last-mentioned fact contains the solution of the problem. The premises from which this reasoning set out were all incorrect – the one red spot on the eleventh segment is likewise a ring-spot, and indeed the most important one of all, being primary, or the first to come into existence. Now all specimens, without exception, possess this first ring-spot, which is useful, and has therefore been called forth by natural selection; it is not inherited, but newly acquired by this species; at least, if the explanation of these spots which I have previously offered is correct.
The primary pair of spots may have been transferred from this to later species by heredity; and since, in all segmented animals there is a tendency for the peculiarities of one segment to be repeated on the others, this repetition must have occurred with greater frequency and more completely in the later species – the more so if the process were favoured by natural selection, i. e. if the row of ring-spots which originated in this manner could in any way be turned to the use of the species.
In Hippophaës itself there must also be a tendency to the formation of secondary ring-spots, and indeed in a number of specimens we actually see series of such ring-spots, the latter being present in varying numbers, and in very different states of development. The fact that the ring-spots have not become a constant and well-developed character, is simply explained by the circumstance that as such they would have endangered the existence of the species.
In this case there is therefore no necessity for assuming a phyletic vital force. The ring-spots of the genus Deilephila rather furnish us with an excellent explanation of a fact which might otherwise have been adduced in support of a phyletic vital force, viz., the strict uniformity in the development of larval markings.
Before I had been led to the discovery, by the study of the marking and development of Hippophaës, that the spots of the genus Deilephila originated on one segment only, from which they were transferred secondarily to the others, this astonishing regularity appeared to me an incomprehensible problem, which could only be solved by assuming a phyletic vital force. If it be attempted, for the ten species here considered, to construct a genealogical tree based on the supposition that it is the rows of spots which have been inherited in cases where they occur, and not the mere tendency to their production by the transference of the one originally inherited primary spot to the remaining segments, the attempt will fail. The greater number of the species would have to be arranged in one row, since one species always bears a perfected form of marking, which appears in the young stages of the following species. But it is very improbable that nine different species, derived directly the one from the other, would contemporaneously survive.156 One species, D. Vespertilio, could not be inserted at all in the genealogical tree, since it wants one character which occurs in all the other species, viz., the caudal horn, which is absent even in the third stage, and must therefore have been lost at a very early period of the phyletic development, so that we may consider it to be on this account genetically allied to the oldest known form. But the markings of this larva pass through precisely the same stages of development as do those of the other species. Now if the ring-spots were inherited as such, the existence of a hornless species with ring-spots would be an insoluble riddle, and would favour the admission of parallel developmental series, which again could be scarcely otherwise explained than by a “fixed direction of variation.” We have here one of that class of cases which the supporters of a phyletic vital force have already so often made use of in support of their view.
The explanation of such a case —i. e. its reference to known causes of species transformation – is never easy, and is indeed impossible without a precise knowledge of the ontogeny of many species, as well as of the original significance of the characters in question. In the case of the Deilephila larvæ, however, such knowledge is still wanting. It is true that they present us with parallel developmental series, but these do not depend on an unknown phyletic force – the parallelism can be referred to the action of the imperfectly known laws of growth innate in segmented organisms. Because the characters of one segment have a tendency to repeat themselves on the others, from one parent-form possessing ring-spots on one segment only, there may have proceeded several developmental series, all of which developed rows of such spots independently of each other.
From these considerations we may venture to construct the following genealogical tree: —
Possible genealogical tree of the Genus Deilephila
The circles indicate the phyletic stages IV. – VIII.; the eighth is only reached by Nicæa, and is distinguished from the seventh chiefly by the ontogeny, in the third stage of which the seventh phyletic stage is reached, whilst in Euphorbiæ and Dahlii this stage is reached in the fourth ontogenetic stage. The phyletic stages indicated by queries are extinct, and only known through the ontogeny of existing species. It must be understood that this pedigree expresses only the ideal and not the actual relations of the species to one another. Thus, it is possible that Hippophaës is not the parent-form, but an unknown or extinct species, which must, however, have possessed the same marking, and so on.
Four parallel series here proceed from the parent-form Hippophaës; there may have been five, or possibly only three, but the incomplete state of our knowledge of the ontogeny does not permit of any certain conclusion. For the point under consideration this is, however, quite immaterial. The distance from the central point (the parent-form) indicates the grade of phyletic development which the respective species have at present reached.
There is another case which is no less instructive, because it reveals, although in a somewhat different manner, the action of a law of growth innate in the organism itself, but which can nevertheless by no means be regarded as equivalent to a phyletic vital force. I refer to the coloured edges of the oblique stripes which occur in most of the species of the genus Sphinx. It has already been insisted upon in a previous section, that the mode in which this character originates negatives the assumption of a phyletic force, because these coloured edges are gradually built up out of irregularly scattered spots. There is no occasion for a “developmental force” to grope in the dark; if such a power exists, we should expect that it would add new characters to old ones with the precision of a master workman.
If, however, the coloured edges certainly depend on natural selection, this agency causing the scattered spots to coalesce and become linear, we have here the proof that such spots first arose in a precisely similar manner in several species, quite independently of one another – that, in fact, a “fixed direction of variation” in a certain sense exists.
In three species of Smerinthus-larvæ, red spots appear towards the end of the ontogeny; in S. Populi and Ocellatus in only a minority of individuals, and always separate (not coalescent), and in S. Tiliæ in a majority of specimens, the spots frequently becoming fused into one large, single, longish marking. These three species cannot have inherited the spots from a common ancestor, since they are absent in the younger ontogenetic stages, or occur only exceptionally, becoming larger and more numerous in the last stage; they obviously form a character which must be considered as a case of “anticipated development.”
How is it then that three species vary independently of each other in an analogous manner? I know of no other answer to this question than that similar variations must necessarily arise from similar physical constitutions – or, otherwise expressed, the three species have inherited from an unknown parent species, devoid of spots, not this last character itself, but a physical constitution, having a tendency to the formation of red spots on the skin.157 The case offers many analogies to that of the colour varieties of Lacerta Muralis, to which Eimer158 briefly calls attention in his interesting communications on the blue lizard of the Faraglioni Rocks at Capri. The South Italian lizards, although having differently formed skulls, show the same brilliantly coloured varieties as those of North Italy; and Eimer believes that these parallel variations in widely separated localities, some of which have long been isolated, must be referred to a tendency towards fixed directions of variation innate in the constitution of the species.
I long ago insisted159 that it should not be forgotten that natural selection is, in the first place, dependent upon the variations which an organism offers to this agency, and that, although the number of possible variations may be very great for each species, yet this number is by no means to be considered as literally infinite. For every species there may be impossible variations. For this reason I am of opinion that the physical nature of each species is of no less importance in the production of new characters than natural selection, which must always, in the first place, operate upon the results of this physical nature, i. e. upon the variations presented, and can thus call new ones into existence.
It requires but a slight alteration of the definition to make out of this “restricted” or “limited variability,” which is the necessary consequence of the physical nature of each species, a “fixed direction of variation” in the sense of a phyletic vital force. Instead of – the Smerinthus-larvæ show a tendency to produce red spots on the skin, it is only necessary to say – these larvæ tend to produce red borders to the oblique stripes. The latter statement would, however, be incorrect, since the red borders first arose by the coalescence of red spots through the action of natural selection. It is not even correct to say that all the species of Smerinthus show this tendency to produce spots, since this character does not seem to occur either in S. Quercus or S. Tremulæ.
The distinction between the two modes of conception will become clear if we ask, as an example, whether those Chærocampa-larvæ which do not at present possess eye-spots will subsequently acquire these markings, supposing that they maintain their existence on the earth for a sufficient period?
The supporters of a “fixed direction of variation” would answer this question in the affirmative. Ocelli constitute a character which occurs in nearly all the species of the group – they are the goal towards which the phyletic force is urging, and which must sooner or later be reached by each member of the group. On the other hand, I cannot express so decidedly my own opinion, viz., that such complicated characters as the many-coloured oblique stripes or eye-spots are never the results of purely internal forces, but always arise by the action of natural selection, i. e. by the combination of such minute and simple variations as may present themselves. It may be replied that the formation of eye-spots in those species which are at present devoid of them, cannot indeed be considered impossible, but that they would only appear if the constitution of these species had a tendency to give rise to the production of darker spots on the edge of the subdorsal line, and if at the same time, the possession of eye-spots would be of use to the caterpillar under its special conditions of life.
The condition of affairs would be quite different if we were simply concerned with the transference of a character from one segment on which it was already present, to the remaining segments. The transference would, in this case, result from causes purely innate in the organism – from the action of laws of equilibration or of growth (correlation), and the external conditions of life would play only a negative part, since they might prevent the complete reproduction of a character, such, for example, as eye-spots, on all the segments, in cases where it was disadvantageous to the species. The fact that our species of Chærocampa have only faint indications, and not a completely-developed eye-spot, on the remaining segments, may perhaps be explained in this manner. It is conceivable that the two pairs of ocelli on the front segments are more effective as a means of alarm than if the insects were provided with two long rows of such markings; but nothing can be stated with certainty on this point until experiments have been made with caterpillars having rows of eye-spots.
The question raised above – whether the species of Chærocampa at present devoid of eye-spots are to be expected to acquire this character in the course of their further phyletic development – brings with it another point, which cannot be here passed over.
If the utility of the four kinds of markings in their perfected form is demonstrated, their origination through natural selection is not, strictly speaking, thereby proved. It must also be shown that the first rudiments of these characters were also of use to their possessors. The question as to the utility of the “initial stages” of useful characters must here be set at rest.
In the case of markings such as longitudinal and oblique stripes, it is quite evident that the initial stages of these simple characters do not differ greatly from the perfected marking, but this is certainly not the case with eye- and ring-spots. The most light is thrown upon this question by the latter, because a species which has remained at the initial stage of the formation of ring-spots here presents itself for examination, viz. Deilephila Hippophaës.
I have attempted to show that the orange-red spots, which, as a rule, adorn only the eleventh segment, enhance the adaptive colouring of this caterpillar by their resemblance to the berries of the sea-buckthorn, whilst the general surface resembles the leaves in colour. If this be admitted, the origination of these spots by natural selection offers no difficulty, since a smaller spot, or one of a fainter red, must also be of some use to its possessor.
This case is of importance, as showing that a “change of function” may occur in markings, just as it does in certain organs among the most diverse species of animals, in the course of phyletic development. The spots which in Hippophaës are imitations of red berries, in species which have further advanced phyletically play quite another part – they serve as means of alarm, or signals of distastefulness.
It appears to me very improbable, however, that the perfect ocelli of the Chærocampa-larvæ have also undergone such a “functional change” (Dohrn). I rather believe that the first rudiments of these markings produced the same effect as that which they now exercise, viz., terror. We are certainly not so favourably circumstanced in this case in knowing a species which shows the initial steps of this character in its last stage of life; but in the initial steps which the second stage of certain species present, we see preserved the form under which the eye-spots first appeared in the phylogeny, and from this we are enabled to judge with some certainty of the effect which they must have produced at the time.
In the ontogeny of C. Elpenor and Porcellus we see that a small curvature of the subdorsal line first arises, the concavity of which becomes filled with darker green, and soon afterwards with black; the upwardly curved piece of the subdorsal then becomes detached and more completely surrounded by black. The white fragment of the subdorsal which has become separated, in the next place broadens, and a black (dark) pupil appears in its centre.
Now the first rudiments of the eye-spot certainly appear very insignificant in a caterpillar two centimeters long, but we must not forget that in the ancestors of the existing Chærocampa-larvæ this character appeared in the adult state. If we conceive the curvature of the white subdorsal with the underlying dark pigment to be correspondingly magnified, its importance as a means of alarm can scarcely be denied, particularly when we consider that this marking stands on the enlarged fourth segment, which alone invests the caterpillar with a singular, and, to smaller foes, an alarming appearance. We know that in the case of those Chærocampa-larvæ which possess no eye-spots, the distension of this segment is employed against hostile attacks. (See the illustration of Darapsa Chærilus, Pl. IV., Fig. 34.) Those markings which even only remotely resembled an eye must, in such a position, have increased the terrifying action. On these grounds I believe that it may be safely admitted, that this kind of marking possessed the same significance in its initial stages as it now does when fully perfected. No functional change has here taken place.
Among all the facts brought together in the first section I only know of one group of phenomena which at least permit of an attempt to refer them to a phyletic vital force. This is the occurrence of dark ground-colours in adult larvæ which are of light colours in their young condition. I have already attempted to show that in the Chærocampa-larvæ this change of colour depends on a double adaptation, the young caterpillars being adapted to the green colour of the plant and the adults to the soil and dead leaves. This interpretation appears the more correct when we find the same process, viz. the gradual replacement of the original green by brown colours, among species of widely different genera, which, with the dark colouring, possess the necessarily correlated habit of hiding themselves by day when in the adult condition. This is the case with Sphinx Convolvuli, Deilephila Vespertilio, and Acherontia Atropos.
Thus far all has been easily explicable by natural selection; but when we also see a “tendency” to acquire a dark colour in the course of development, in those species which neither conceal themselves nor are adaptively coloured, but are very conspicuously marked – and if, further, it can be shown that these species, such for instance as Deilephila Galii, actually possess immunity from the attacks of foes, – how can this tendency to the formation of a dark colour be otherwise explained than by the admission of a phyletic vital force urging the variations in this direction?
Nevertheless I believe that also on this point an appeal to unknown forces can be dispensed with. In the first place, dark ground-colours can be of use to a species otherwise than as means of adaptation. In D. Galii, as well as in D. Euphorbiæ, the light ring-spots appear rather at their brightest on the pitchy-black ground; and if this caterpillar must (sit venia verbo!) become conspicuous, this purpose would be best attained by acquiring a dark ground-colour, such as that of D. Euphorbiæ.
The tendency, apparently common to all these Sphingidæ, to acquire a dark colour with increasing age, depends therefore on two quite distinct adaptations – first, in species sought by enemies, on an adaptation to the colour of the soil; and secondly, in species rejected by foes, on the endeavour to produce the greatest possible contrast of colour.
Moreover, the supposition from which this last plea for a vital force set out is not universally correct, since there are species, such for instance as D. Nicæa, which never acquire a dark colour; and in D. Galii also, although all the individuals abandon the protective green of the young stages, they by no means all acquire a dark hue in exchange for this colour; many individuals in their light ochreous-yellow colouring rather strikingly resemble the snake-like caterpillar of D. Nicæa.
