That closely similar structures may arise without a genetic relationship has been lately well urged by Mr. Ray Lankester.[159] He has brought {153} this notion forward even as regards the bones of the skull in osseous fishes and in mammals. He has done so on the ground that the probable common ancestor of mammals and of osseous fishes was a vertebrate animal of so low a type that it could not be supposed to have possessed a skull differentiated into distinct bony elements--even if it was bony at all. If this was so, then the cranial bones must have had an independent origin in each cla.s.s, and in this case we have the most strikingly harmonious and parallel results from independent actions. For the bones of the skull in an osseous fish are so closely conformed to those of a mammal, that "both types of skull exhibit many bones in common," though "in each type some of these bones acquire special arrangements and very different magnitudes."[160] And no investigator of h.o.m.ologies doubts that a considerable number of the bones which form the skull of any osseous fish are distinctly h.o.m.ologous with the cranial bones of man. The occipital, the parietal, and frontal, the bones which surround the internal ear, the vomer, the premaxilla, and the quadrate bones, may be given as examples.
Now, if such close relations of h.o.m.ology can be brought about independently of any but the most remote genetic affinity, it would be rash to affirm dogmatically that there is any impossibility in the independent origin of such forms as centetes and solenodon, or of genetically distinct batrachians, as similar to each other as are some of the frogs of South America and of Europe. At the same time such phenomena must at present be considered as very improbable, from the action of ancestral habit, as before stated.
We have seen, then, that the geographical distribution of animals presents difficulties, though not insuperable ones, for the Darwinian hypothesis.
If, however, other reasons against it appear of any weight--if, especially, there is reason to believe that geological time has not been {154} sufficient for it, then it will be well to bear in mind the facts here enumerated. These facts, however, are not opposed to the doctrine of evolution; and if it could be established that closely similar forms had really arisen in complete independence one of the other, they would rather tend to strengthen and to support that theory. [Page 155]
CHAPTER VIII.
h.o.m.oLOGIES.
Animals made-up of parts mutually related in various ways.--What h.o.m.ology is.--Its various kinds.--Serial h.o.m.ology.--Lateral h.o.m.ology.--Vertical h.o.m.ology.--Mr. Herbert Spencer"s explanations.--An internal power necessary, as shown by facts of comparative anatomy.--Of teratology.--M. St. Hilaire.--Professor Burt Wilder.--Foot-wings.--Facts of pathology.--Mr. James Paget.--Dr.
William Budd.--The existence of such an internal power of individual development diminishes the improbability of an a.n.a.logous law of specific origination.
That concrete whole which is spoken of as "an individual" (such, _e.g._, as a bird or a lobster) is formed of a more or less complex aggregation of parts which are actually (from whatever cause or causes) grouped, together in a harmonious interdependency, and which have a mult.i.tude of complex relations amongst themselves.
The mind detects a certain number of these relations as it contemplates the various component parts of an individual in one or other direction--as it follows up different lines of thought. These perceived relations, though subjective, _as relations_, have nevertheless an objective foundation as real parts, or conditions of parts, of real wholes; they are, therefore, true relations, such, _e.g._, as those between the right and left hand, between the hand and the foot, &c.
The component parts of each concrete whole have also a relation of resemblance to the parts of other concrete wholes, whether of the same{156} or of different kinds, as the resemblance between the hands of two men, or that between the hand of a man and the fore-paw of a cat.
Now, it is here contended that the relationships borne one to another by various component parts, imply the existence of some innate, internal condition, conveniently spoken of as a power or tendency, which is quite as mysterious as is any innate condition, power, or tendency, resulting in the orderly evolution of successive specific manifestations. These relationships, as also this developmental power, will doubtless, in a certain sense, be somewhat further explained as science advances. But the result will be merely a shifting of the inexplicability a point backwards, by the intercalation of another step between the action of the internal condition or power and its external result. In the meantime, even if by "Natural Selection" we could eliminate the puzzles of the "origin of species," yet other phenomena, not less remarkable (namely, those noticed in this chapter), would still remain unexplained and as yet inexplicable.
It is not improbable that, could we arrive at the causes conditioning all the complex inter-relations between the several parts of one animal, we should at the same time obtain the key to unlock the secrets of specific origination.
It is desirable, then, to see what facts there are in animal organization which point to innate conditions (powers and tendencies), as yet unexplained, and upon which the theory of "Natural Selection" is unable to throw any explanatory light.
The facts to be considered are the phenomena of "h.o.m.ology," and especially of serial, bilateral, and vertical h.o.m.ology.
The word "h.o.m.ology" indicates such a relation between two parts that they may be said in some sense to be "the same," or at least "of similar nature." This similarity, however, does not relate to the _use_ to which parts are put, but only to their relative position with regard to other parts, or to their mode of origin. There are many kinds of {157} h.o.m.ology,[161] but it is only necessary to consider the three kinds above enumerated.
[Ill.u.s.tration: WINGBONES OF PTERODACTYLE, BAT, AND BIRD.]
The term "h.o.m.ologous" may be applied to parts in two individual animals of different kinds, or to different parts of the same individual. Thus "the right and left hands," or "joints of the backbone," or "the teeth of the two jaws," are h.o.m.ologous parts of the same individual. But the arm of a man, the fore-leg of the horse, the paddle of the whale, and the wing of the bat and the bird are all also h.o.m.ologous parts, yet of another kind, _i.e._ they are the same parts existing in animals of different species.
On the other hand, the wing of the humming-bird and the wing of the humming-bird moth are not h.o.m.ologous at all, or in any sense; for the resemblance between them consists solely in the use to which they are put, and is therefore only a relation of _a.n.a.logy_. There is no relation of _h.o.m.ology_ between them, because they have no common resemblance as to their relations to surrounding parts, or as to their mode of origin.
Similarly, there is no h.o.m.ology between the wing of the bat and that {158} of the flying-dragon, for the latter is formed of certain ribs, and not of limb bones.
[Ill.u.s.tration: SKELETON OF THE FLYING DRAGON.
(Showing the elongated ribs which support the flitting organ.)]
h.o.m.ology may be further distinguished into (1) a relationship which, on evolutionary principles, would be due to descent from a common ancestor, as the h.o.m.ological relation between the arm-bone of the horse and that of the ox, or between the singular ankle bones of the two lemurine genera, cheirogaleus and galago, and which relation has been termed by Mr. Ray Lankester "h.o.m.ogeny;"[162] and (2) a relationship induced, not derived--such as exists between parts closely similar in relative position, but with no genetic affinity, or only a remote one, as the h.o.m.ological relation between the chambers of the heart of a bat and those of a {159} bird, or the similar teeth of the thylacine and the dog before spoken of.
For this relationship Mr. Bay Lankester has proposed the term "h.o.m.oplasy."
[Ill.u.s.tration: TARSAL BONES OF DIFFERENT LEMUROIDS.
(Right tarsus of Galago; left tarsus of Cheirogaleus.)]
[Ill.u.s.tration: A CENTIPEDE.]
"Serial h.o.m.ology" is a relation of resemblance existing between two or more parts placed in series one behind the other in the same individual.
Examples of such h.o.m.ologues are the ribs, or joints of the backbone of{160} a horse, or the limbs of a centipede. The latter animal is a striking example of serial h.o.m.ology. The body (except at its two ends) consists of a longitudinal series of similar segments. Each segment supports a pair of limbs, and the appendages of all the segments (except as before) are completely alike.
[Ill.u.s.tration: SQUILLA.]
A less complete case of serial h.o.m.ology is presented by Crustacea (animals of the crab cla.s.s), notably by the squilla and by the common lobster. In the latter animal we have a six-jointed abdomen (the so-called tail), {161} in front of which is a large solid ma.s.s (the cephalo-thorax), terminated anteriorly by a jointed process (the rostrum). On the under surface of the body we find a quant.i.ty of moveable appendages. Such are, _e.g._, feelers (Fig. 9), jaws (Figs. 6, 7, and 8), foot-jaws (Fig. 5), claws and legs (Figs. 3 and 4), beneath the cephalo-thorax; and flat processes (Fig. 2), called "swimmerets," beneath the so-called tail or abdomen.
[Ill.u.s.tration: PART OF THE SKELETON OF THE LOBSTER.]
Now, these various appendages are distinct and different enough as we {162} see them in the adult, but they all appear in the embryo as buds of similar form and size, and the thoracic limbs at first consist each of two members, as the swimmerets always do.
This shows what great differences may exist in size, in form, and in function, between parts which are developmentally the same, for all these appendages are modifications of one common kind of structure, which becomes differently modified in different situations; in other words, they are serial h.o.m.ologues.
The segments of the body, as they follow one behind the other, are also serially alike, as is plainly seen in the abdomen or tail. In the cephalo-thorax of the lobster, however, this is disguised. It is therefore very interesting to find that in the other crustacean before mentioned, the squilla, the segmentation of the body is more completely preserved, and even the first three segments, which go to compose the head, remain permanently distinct.
[Ill.u.s.tration: SPINE OF GALAGO ALLENII.]
Such an obvious and unmistakeable serial repet.i.tion of parts does not obtain in the highest, or backboned animals, the Vertebrata. Thus in man and other mammals, nothing of the kind is _externally_ visible, and we have to penetrate to his skeleton to find such a series of h.o.m.ologous parts.
There, indeed, we discover a number of pairs of bones, each pair so obviously resembling the others, that they all receive a common name--the ribs. There also (_i.e._ in the skeleton) we find a still more remarkable series of similar parts, the joints of the spine or backbone (vertebrae), which are admitted by all to possess a certain community of structure.{163}
It is in their limbs, however, that the Vertebrata present the most obvious and striking serial h.o.m.ology--almost the only serial h.o.m.ology noticeable externally.
The facts of serial h.o.m.ology seem hardly to have excited the amount of interest they certainly merit.
Very many writers, indeed, have occupied themselves with investigations and speculations as to what portions of the leg and foot answer to what parts of the arm and hand, a question which has only recently received a more or less satisfactory solution through the successive concordant efforts of Professor Humphry,[163] Professor Huxley,[164] the Author of this work,[165] and Professor Flower.[166] Very few writers, however, have devoted much time or thought to the question of serial h.o.m.ology in general.
Mr. Herbert Spencer, indeed, in his very interesting "First Principles of Biology," has given forth ideas on this subject, which are well worthy careful perusal and consideration, and some of which apply also to the other kinds of h.o.m.ology mentioned above. He would explain the serial h.o.m.ologies of such creatures as the lobster and centipede thus: Animals of a very low grade propagate themselves by spontaneous fission. If certain creatures found benefit from this process of division remaining incomplete, such creatures (on the theory of "Natural Selection") would transmit their selected tendency to such incomplete division to their posterity. In this way, it is conceivable, that animals might arise in the form of long chains of similar segments, each of which chains would consist of a number of imperfectly separated individuals, and be equivalent to a series of separate individuals belonging to kinds in which the fission was complete.
In other words, Mr. Spencer would explain it as the coalescence of {164} organisms of a lower degree of aggregation in one longitudinal series, through survival of the fittest aggregations. This may be so. It is certainly an ingenious speculation, but facts have not yet been brought forward which demonstrate it. Had they been so, this kind of serial h.o.m.ology might be termed "h.o.m.ogenetic."
The other kind of serial repet.i.tions, namely, those of the vertebral column, are explained by Mr. Spencer as the results of alternate strains and compressions acting on a primitively h.o.m.ogeneous cylinder. The serial h.o.m.ology of the fore and hind limbs is explained by the same writer as the result of a similarity in the influences and conditions to which they are exposed. Serial h.o.m.ologues so formed might be called, as Mr. Ray Lankester has proposed, "h.o.m.oplastic." But there are, it is here contended, abundant reasons for thinking that the predominant agent in the production of the h.o.m.ologies of the limbs is an _internal_ force or tendency. And if such a power can be shown to be necessary in this instance, it may also be legitimately used to explain such serial h.o.m.ologies as those of the centipede"s segments and of the joints of the backbone. At the same time it is not, of course, pretended that external conditions do not contribute their own effects in addition. The presence of this internal power will be rendered more probable if valid arguments can be brought forward against the explanations which Mr. Herbert Spencer has offered.
_Lateral h.o.m.ology_ (or bilateral symmetry) is the resemblance between the right and left sides of an animal, or of part of an animal; as, _e.g._, between our right hand and our left. It exists more or less at one or other time of life in all animals, except some very lowly organized creatures. In the highest animals this symmetry is laid down at the very dawn of life, the first trace of the future creature being a longitudinal streak--the embryonic "primitive groove." This kind of h.o.m.ology is explained by Mr.
Spencer as the result of the similar way in which conditions affect {165} the right and left sides respectively.
[Ill.u.s.tration: VERTEBRae OF AXOLOTL.]
_Vertical h.o.m.ology_ (or vertical symmetry) is the resemblance existing between parts which are placed one above the other beneath. It is much less general and marked than serial, or lateral h.o.m.ology. Nevertheless, it is plainly to be seen in the tail region of most fishes, and in the far-extending dorsal (back) and ventral (belly) fins of such kinds as the sole and the flounder.
It is also strikingly shown in the bones of the tail of certain efts, as in _Chioglossa_, where the complexity of the upper (neural) arch is closely repeated by the inferior one. Again, in _Spelerpes rubra_, where almost vertically ascending articular processes above are repeated by almost vertically descending articular processes below. Also in the axolotl, where there are douple pits, placed side by side, not only superiorly but at the same time inferiorly.[167]
This kind of h.o.m.ology is also explained by Mr. Spencer as the result of the similarity of conditions affecting the two parts. Thus he explains the very general absence of symmetry between the dorsal and ventral surfaces of animals by the different conditions to which these two surfaces are respectively exposed, and in the same way he explains the asymmetry of the flat-fishes (_Pleuronectidae_), of snails, &c.
Now, first, as regards Mr. Spencer"s explanation of animal forms by means of the influence of external conditions, the following observations may be made. Abundant instances are brought forward by him of admirable adaptation of structure to circ.u.mstances, but as to the immense majority of these it is very difficult, if not impossible, to see _how_ external conditions{166} can have produced, or even tended to have produced them. For example, we may take the migration of one eye of the sole to the other side of its head. What is there here either in the darkness, or the friction, or in any other conceivable external cause, to have produced the first beginning of such an unprecedented displacement of the eye? Mr. Spencer has beautifully ill.u.s.trated that correlation which all must admit to exist between the forms of organisms and their surrounding external conditions, but by no means proved that the latter are _the cause_ of the former.
[Ill.u.s.tration: PLEURONECTIDae, WITH THE PECULIARLY PLACED EYE IN DIFFERENT POSITIONS.]
Some internal conditions (or in ordinary language some internal power and force) must be conceded to living organisms, otherwise incident forces must act upon them and upon non-living aggregations of matter in the same way and with similar effects.
If the mere presence of these incident forces produces so ready a response in animals and plants, it must be that there are, in their case, conditions disposing and enabling them so to respond, according to the old maxim, _Quicquid recipitur, recipitur ad modum recipientis_, as the same rays of light which bleach a piece of silk, blacken nitrate of silver. If, therefore, we attribute the forms of organisms to the action of {167} external conditions, _i.e._ of incident forces on their modifiable structure, we give but a partial account of the matter, removing a step back, as it were, the action of the internal condition, power, or force which must be conceived as occasioning such ready modifiability. But indeed it is not at all easy to see how the influence of the surface of the ground or any conceivable condition or force can produce the difference which exists between the ventral and dorsal shields of the carapace of a tortoise, or by what differences of merely external causes the ovaries of the two sides of the body can be made equal in a bat and unequal in a bird.
[Ill.u.s.tration: AN ECHINUS, OR SEA-URCHIN.