Even more striking than the substitutions of organs are the suppressions of organs. Mr. Darwin names some cases as "extremely curious; for instance, the presence of teeth in fœtal whales, which when grown up have not a tooth in their heads;… It has even been stated on good authority that rudiments of teeth can be detected in the beaks of certain embryonic birds." Irreconcilable with any teleological theory, these facts do not even harmonize with the theory of fixed types which are maintained by the development of all the typical parts, even where not wanted; seeing that the disappearance of these incipient organs during fœtal life spoils the typical resemblance. But while to other hypotheses these facts are stumbling-blocks, they yield strong support to the hypothesis of evolution.
Allied to these cases, are the cases of what has been called retrograde development. Many parasitic creatures and creatures which, after leading active lives for a time, become fixed, lose, in their adult states, the limbs and senses they had when young. It may be alleged, however, that these creatures could not secure the habitats needful for them, without possessing, during their larval stages, eyes and swimming appendages which eventually become useless; that though, by losing these, their organization retrogresses in one direction, it progresses in another direction; and that, therefore, they do not exhibit the needless development of a higher type on the way to a lower type. Nevertheless there are instances of a descent in organization, following an apparently-superfluous ascent. Mr. Darwin says that in some genera of cirripedes, "the larvæ become developed either into hermaphrodites having the ordinary structure, or into what I have called complemental males, and in the latter, the development has assuredly been retrograde; for the male is a mere sack, which lives for a short time, and is destitute of mouth, stomach, or other organ of importance, excepting for reproduction."
§ 130a. But now let us contemplate more closely the energies at work in the unfolding embryo, or rather the energies which the facts appear to imply.
Whatever natures we ascribe to the hypothetical units proper to each kind of organism, we must conclude that from the beginning of embryonic development, they have a proclivity towards the structure of that organism. Because of their phylogenetic origin, they must tend towards the form of the primitive type; but the superposed modifications, conflicting with their initial tendency, must cause a swerving towards each successively higher type. To take an illustration: – If in the germ-plasm out of which will come a vertebrate animal there is a proclivity towards the primitive piscine form, there must, if the germ-plasm is derived from a mammal, be also from the outset a proclivity towards the mammalian form. While the initial type tends continually to establish itself the terminal type tends also to establish itself. The intermediate structures must be influenced by their conflict, as well as by the conflict of each with the proclivities towards the amphibian and reptilian types. This complication of tendencies is increased by the intervention of several other factors.
There is the factor of economy. An embryo in which the transformations have absorbed the smallest amount of energy and wasted the smallest amount of matter, will have an advantage over embryos the transformations of which have cost more in energy and matter: the young animal will set out with a greater surplus of vitality, and will be more likely than others to live and propagate. Again, in the embryos of its descendants, inheriting the tendency to economical transformation, those which evolve at the least cost will thrive more than the rest and be more likely to have posterity. Thus will result a continual shortening of the processes. We can see alike that this must take place and that it does take place. If the whole series of phylogenetic changes had to be repeated – if the embryo mammal had to become a complete fish, and then a complete amphibian, and then a complete reptile, there would be an immense amount of superfluous building up and pulling down, entailing great waste of time and of materials. Evidently these abridgments which economy entails, necessitate that unfolding embryos bear but rude resemblances to lower types ancestrally passed through – vaguely represent their dominant traits only.
From this principle of economy arise several derivative principles, which may be best dealt with separately.
§ 130b. In some cases the substitution of an abridged for an unabridged course of evolution causes the entire disappearance of certain intermediate forms. Structural arrangements once passed through during the unfolding are dropped out of the series.
In the evolution of these embryos with which there is not laid up a large amount of food-yolk there occurs at the outset a striking omission of this kind. When, by successive fissions, the fertilized cell has given rise to a cluster of cells constituting a hollow sphere, known as a blastula, the next change under its original form is the introversion of one side, so as to produce two layers in place of one. An idea of the change may be obtained by taking an india-rubber ball (having a hole through which the air may escape) and thrusting in one side until its anterior surface touches the interior surface of the other side. If the cup-shaped structure resulting be supposed to have its wide opening gradually narrowed, until it becomes the mouth of an internal chamber, it will represent what is known as a gastrula– a double layer of cells, of which the outer is called epiblast and the inner hypoblast (answering to ectoderm and endoderm) inclosing a cavity known as the archenteron, or primitive digestive sac. But now in place of this original mode of forming the gastrula, there occurs a mode known as delamination. Throughout its whole extent the single layer splits so as to become a double layer – one sphere of cells inclosing the other; and after this direct formation of the double layer there is a direct formation of an opening through it into the internal cavity. There is thus a shortening of the primitive process: a number of changes are left out.
Often a kindred passing over of stages at later periods of development may be observed. In certain of the Mollusca, as the Patella chiton, the egg gives origin to a free-swimming larva known as a trochosphere, from which presently comes the ordinary molluscous organization. In the highest division of the Molluscs, however, the Cephalopods, no trochosphere is formed. The nutritive matter laid up in the egg is used in building up the young animal without any indication of an ancestral larva.
§ 130c. Among principles derived from the principle of economy is the principle of pre-adaptation – a name which we may appropriately coin to indicate an adaptation made in advance of the time at which it could have arisen in the course of phylogenetic history.
How pre-adaptation may result from economy will be shown by an illustration which human methods of construction furnish. Let us assume that building houses of a certain type has become an established habit, and that, as a part of each house, there is a staircase of given size. And suppose that in consequence of changed conditions – say the walling in of the town, limiting the internal space and increasing ground-rents – it becomes the policy to build houses of many stories, let out in flats to different tenants. For the increased passing up and down, a staircase wider at its lower part will be required. If now the builder, when putting up the ground floor, follows the old dimensions, then after all the stories are built, the lower part of the staircase, if it is to yield equal facilities for passage, must be reconstructed. Instead of a staircase adapted to those few stories which the original type of house had, economy will dictate a pre-adaptation of the staircase to the additional stories.
On carrying this idea with us, we shall see that if from some type of organism there is evolved a type in which enlargement of a certain part is needed to meet increased functions, the greater size of this part will begin to show itself during early stages of unfolding. That unbuilding and rebuilding which would be needful were it laid down of its original size, will be made needless if from the beginning it is laid down of a larger size. Hence, in successive generations, the greater prosperity and multiplication of individuals in which this part is at the outset somewhat larger than usual, must eventually establish a marked excess in its development at an early stage. The facts agree with this inference.
Referring to the contrasts between embryos, Mr. Adam Sedgwick says that "a species is distinct and distinguishable from its allies from the very earliest stages." Whereas, according to the law of von Baer, "animals so closely allied as the fowl and duck would be indistinguishable in the early stages of development," "yet I can distinguish a fowl and a duck embryo on the second day by the inspection of a single transverse section through the trunk." This experience harmonizes with the statement of the late Prof. Agassiz, that in some cases traits characterizing the species appear at an earlier period than traits characterizing the genus.
Similar in their implications are the facts recently published by Dr. E. Mehnert, concerning the feet of pentadactyle vertebrates. A leading example is furnished by the foot in the struthious birds. Out of the original five digits the two which eventually become large while the others disappear, soon give sign of their future predominance: their early sizes being in excess of those required for the usual functional requirements in birds, and preparing the way for their special requirements in the struthious birds. Dr. Mehnert shows that a like lesson is given by the relative developments of legs and wings in these birds. Ordinarily in vertebrates the fore limbs grow more rapidly than the hind limbs; but in the ostrich, in which the hind limbs or legs have to become so large while the wings are but little wanted, the leg development goes in advance of the wing-development in early embryonic stages: there is a pre-adaptation.
Much more striking are examples furnished by creatures whose modes of existence require that they shall have enormous fertility – require that the generative system shall be very large. Ordinarily the organs devoted to maintenance of the race develop later than the organs devoted to maintenance of the individual. But this order is inverted in certain Entozoa. To these creatures, imbedded in nutritive matters, self-maintenance cost nothing, and the structures devoted to it are relatively of less importance than the structures devoted to race-maintenance, which, to make up for the small chance any one germ has of getting into a fit habitat, have to produce immense numbers of germs. Here the rudiments of the generative systems are the first to become visible – here, in virtue of the principle of pre-adaptation, a structure belonging to the terminal form asserts itself so early in the developmental process as almost to obliterate the structure of the initial form.
It may be that in some cases where the growth of certain organs goes in advance of the normal order, the element of time comes into play – the greater time required for construction. To elucidate this let us revert to our simile. Suppose that the staircase above instanced, or at any rate its lower part, is required to be of marble with balusters finely carved. If this piece of work is not promptly commenced and pushed on fast, it will not be completed when the rest of the house is ready: workmen and tools will still block it up at a time when it should be available. Similarly among the parts of an unfolding embryo, those in which there is a great deal of constructive work must early take such shape as will allow of this. Now of all the tissues the nervous tissue is that which takes longest to repair when injured; and it seems a not improbable inference that it is a tissue which is slower in its histological development than others. If this be so, we may see why, in the embryos of the higher vertebrates, the central nervous system quickly grows large in comparison to the other systems – why by pre-adaptation the brain of a chick develops in advance of other organs so much more than the brain of a fish.
§ 130d. Yet another complication has to be noted. From the principle of economy, it seems inferable that decrease and disappearance of organs which were useful in ancestral types but have ceased to be useful, should take place uniformly; but they do not. In the words of Mr. Adam Sedgwick, "some ancestral organs persist in the embryo in a functionless rudimentary (vestigial) condition and at the same time without any reference to adult structures, while other ancestral organs have disappeared without leaving a trace."46 This anomaly is rendered more striking when joined with the fact that some of the structures which remain conspicuous are relatively ancient, while some which have been obliterated are relatively modern —e. g., "gill slits [which date back to the fish-ancestor], have been retained in embryology, whereas other organs which have much more recently disappeared, e. g. teeth of birds, fore-limbs of snakes [dating back to the reptile ancestor], have been entirely lost."47 Mr. Sedgwick ascribes these anomalies to the difference between larval development and embryonic development, and expresses his general belief thus: —
"The conclusion here reached is that, whereas larval development must retain traces (it may be very faint) of ancestral stages of structure because they are built out of ancestral stages, embryonic development need not necessarily do so, and very often does not; that embryonic development in so far as it is a record at all, is a record of structural features of previous larval stages. Characters which disappear during free life disappear also in the embryo, but characters which though lost by the adult are retained in the larva may ultimately be absorbed into the embryonic phase and leave their traces in embryonic development."48
To set forth the evidence justifying this view would encumber too much the general argument. Towards elucidation of such irregularities let me name two factors which should I think be taken into account.
Abridgment of embryonic stages cannot go on uniformly with all disused organs. Where an organ is of such size that progressive diminution of it will appreciably profit the young animal, by leaving it a larger surplus of unused material, we may expect progressive diminution to occur. Contrariwise, if the organ is relatively so small that each decrease will not, by sensibly increasing the reserve of nutriment, give the young animal an advantage over others, decrease must not be looked for: there may be a survival of it even though of very ancient origin.
Again, the reduction of a superfluous part can take place only on condition that the economy resulting from each descending variation of it, is of greater importance than are the effects of variations simultaneously occurring in other parts. If by increase or decrease of any other parts of the embryo, survival of the animal is furthered in a greater degree than by decrease of this superfluous part, then such decrease is unlikely; since it is illegitimate to count upon the repeated concurrence of favourable variations in two or more parts which are independent. So that if changes of an advantageous kind are going on elsewhere in the embryo a useless part may remain long undiminished.
Yet another cause operates, and perhaps cooperates. Embryonic survival of an organ which has become functionless, may readily happen if, during subsequent stages of development, parts of it are utilized as parts of other organs. In the words of Mr. J. T. Cunningham: —
"It seems to be a general fact that a structure which in metamorphosis disappears completely may easily be omitted altogether in embryonic development, while one which is modified into something else continues to pass more or less through its original larval condition." (Science Progress, July, 1897, p. 488.)
One more factor of considerable importance should be taken into account. A disused organ which entails evil because construction of it involves needless cost, may entail further evil by being in the way. This, it seems to me, is the reason why the fore-limbs of snakes have disappeared from their embryos. When the long-bodied lizard out of which the ophidian type evolved, crept through stiff herbage, and moved its head from side to side to find openings, there resulted alternate bends of its body, which were the beginnings of lateral undulations; and we may easily see that in proportion as it thus progressed by insinuating itself through interstices, the fore-limbs, less and less used for walking, would be more and more in the way; and the lengthening of the body, increasing the undulatory motion and decreasing the use of the fore-limbs, would eventually make them absolute impediments. Hence besides the benefit in economy of construction gained by embryos in which the fore-limbs were in early stages a little less developed than usual, they would gain an advantage by having, when mature, smaller fore-limbs than usual, leading to greater facility of locomotion. There would be a double set of influences causing, through selection, a comparatively rapid decrease of these appendages. And we may I think see also, on contemplating the kind of movement, that the fore-limbs would be more in the way than the hind limbs, which would consequently dwindle with such smaller rapidity as to make continuance of the rudiments of them comprehensible.
§ 131-132. So that while the embryonic law enunciated by von Baer is in harmony with the hypothesis of evolution, and is, indeed, a law which this hypothesis implies, the nonconformities to the law are also interpretable by this hypothesis.
Parallelism between the courses of development in species allied by remote ancestry, is liable to be variously modified in correspondence with the later ancestral forms passed through after divergence of such species. The substitution of a direct for an indirect process of formation, which we have reason to believe will show itself, must obscure the embryonic history. And the principle of economy which leads to this substitution produces effects that are very irregular and uncertain in consequence of the endlessly varied conditions. Thus several causes conspire to produce deviations from the general law.
Let it be remarked, finally, that the ability to trace out embryologic kinships and the inability to do this, occur just where, according to the hypothesis of Evolution, they should occur. We saw in § 100a that zoologists are agreed in grouping animals into some 17 phyla —Mollusca, Arthropoda, Echinodermata, &c. – each of which includes a number of classes severally sub-divided into orders, genera, species. All the members of each phylum are so related embryologically, that the existence of a common ancestor of them in the remote past is considered certain. But when it comes to the relations among the archaic ancestors, opinion is unsettled. Whether, for instance, the primitive Chordata, out of which the Vertebrata emerged, have molluscan affinities or annelidan affinities, is still a matter in dispute. With regard to the origins of various other types no settled conclusions are held. Now it is clear that on tracing down each branch of the great genealogical tree, kinships would be much more manifest among the recently-differentiated forms than among those forms which diverged from one another in the earliest stages of organic life, and had separated widely before any of the types we now know had come into existence.
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