On the Genesis of Species

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.-M

a more or less complex aggregation of parts which are actually (from whatever cause or causes) grouped, to

up different lines of thought. These perceived relations, though subjective, as relations, have nevertheless an objective foundation as real parts, or cond

s of other concrete wholes, whether of the same or of different kinds, as the resemblance

elopmental 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

point to innate conditions (powers and tendencies), as yet unexplained, and upon

omena of "homology," and especially of

ature." This similarity, however, does not relate to the use to which parts are put, but only to their relative position with regard to other p

PTERODACTYLE,

or "joints of the backbone," or "the teeth of the two jaws," are homologous parts of the same individual. But the arm of a man, the fore-leg of the horse, the paddle of

o which they are put, and is therefore only a relation of analogy. There is no relation of homology between them, because they have no common resemblance as to their relations to surrounding

F THE FLYI

ed ribs which suppor

of the two lemurine genera, cheirogaleus and galago, and which relation has been termed by Mr. Ray Lankester "homogeny;"[162] and (2) a relationship induced, not derived-such as exists between parts closely similar in relative position, but with no genetic affi

OF DIFFEREN

alago; left tarsus

NTIP

re the ribs, or joints of the backbone of a horse, or the limbs of a centipede. The latter animal is a striking example of serial homology. The body (except at its two ends)

UI

l), in front of which is a large solid mass (the cephalo-thorax), terminated anteriorly by a jointed process (the rostrum). On the under surface of the body we find a quantity of moveable appendages. Such are, e.g.

SKELETON OF

he adult, but they all appear in the embryo as buds of similar form and size, and t

developmentally the same, for all these appendages are modifications of one common kind of structure,

ax of the lobster, however, this is disguised. It is therefore very interesting to find that in the other crustacean before mentioned, the squilla, th

GALAGO

backboned animals, the Vertebrata. Thus in man and other mammals, nothing of the kind is externa

eive a common name-the ribs. There also (i.e. in the skeleton) we find a still more remarkable series of similar parts

esent the most obvious and striking serial homology-a

hardly to have excited the amoun

on this subject, which are well worthy careful perusal and consideration, and some of which apply also to the other kinds of homology mentioned above. He would explain the serial homologies 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

omologues so formed might be called, as Mr. Ray Lankester has proposed, "homoplastic." But there are, it is here contended, abundant reasons for thinking that the predominant agent in the production of the homologies 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 se

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 bein

R? OF

h. It is much less general and marked than serial, or lateral homology. Nevertheless, it is plainly to be seen in the tail reg

d by the inferior one. Again, in Spelerpes rubra, where almost vertically ascending articular processes above are repeated by almost vertically descending

plains the very general absence of symmetry between the dorsal and ventral surfaces of animals by the different conditions to which thes

cult, if not impossible, to see how external conditions 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

PECULIARLY PLACED EYE

ust be conceded to living organisms, otherwise incident forces must act upon them

If, therefore, we attribute the forms of organisms to the action of 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 i

US, OR S

removed fr

lexities of form which can hardly be thought to be due to other than internal causes. The same may be said of the great group of Echinoderms, with their amazing variety of component parts. If then internal forces can so build up the most varied structures, they are surely capable of producing the serial, lateral, and vertical symmetries which higher animal forms exhibit. Mr. Spencer is the more bound to admit this, inasmuch as in his doctrine of "physiological units" he main

xplain the peculiar condition presented by Syllis and some other annelids, where a new head is formed at intervals in certain segments of the body. Here there is evidently an innate tendency to the development at intervals of a complex whole. It is not the budding out or spontaneous fission of certain segments, but the transformation in a definite and very peculiar manner of parts which already exist into other and more complex parts. Again, the processes of

DIVIDING SP

med towards the hinder end

se to secondary larv? within it, which develop and burst the body of the primary larva. The secon

ous individuals, tends to invalidate the argument that the increa

uccessive chambers of the orthoceratid?. Nor are parts of a series less serial, because arranged spirally, as in most gasteropods. Mr. Spencer observes of the molluscous as of the vertebrate animal, "You cannot cut it into transverse slices, each of which contains a digestive organ, a respiratory organ,

gregation, we ought to find this process much less perfect in the oldest form. But a complete development, such as already obtains in the lobster, &c., was reached by the Eurypt

LOB

is exemplified by the backbone of man, there are also se

body, we ought to find the segmentation much more complete laterally than on the dorsal and ventral aspects of the spinal column. Nevertheless, in those species which

t Dr. Günther informs me that the sluggishness of the common tope (Galeus vulgaris) is much like that of the sturgeon, and yet the bodies of its vertebr? are

Müller describes them as possessing a persistent chorda dorsalis.[174] It may be they have th

ed must prevent any excessive flexion of the body, and yet its vertebral column presents a degre

t forward by Professor Huxley,[175] is probably formed of a number of coalesced segments, of some of which the trabecul? cranii and the mandibular and hyoidean arches are indications. What is, perhaps, most remarkable however is, that the

of serial homology. The explanations suggested are very ingenious, yet repose upon a very small basis of fact. Not

nly from facts of comparative anatomy, but also from those of teratology[176] and pathology. These facts appear to show, not only that there are homological internal relations, but that they are so strong and energetic as to re-assert and re-exhibit themselves in creatures which, on th

f these phenomena other than the imposition on them of the name "laws of correlation;" and indeed he says, "The nature of the bond of correlation is frequently quite obscure." Now, it is surely

VARK (ORY

GOLIN (

rmal than are the American anteaters as regards their dermal covering, in their dentition are less so. The Cape ant-eater, on the other hand, the Aard-vark (Orycteropus), has teeth formed on a type quite different from that existing in any other mammal; yet its hairy coat is not known to exhibit any such strange peculiarity. Again, those remarkable scaly ant-eaters of the Old World-the pangolins (Manis)-stand alone amongst mammals as regards their dermal covering; having been classed with lizards by early naturalists on account of their clothing of scales, yet their mouth is like that of the hairy ant-eaters of the New World. On the other hand, the duck-billed platypus o

GO

, and vertical homologies, without the action of some special innate power or tendency so to build up, possessed by the organism itself in each case. By "special tendency" is meant one the

is fin is supposed to have had a marginal external (radial) series of cartilages, each of which supported a series of secondary cartilages, starting from the inner (ulnar) side of the distal part of the supporting marginal piece. The root marginal piece

(radial or marginal) side of the paddle in the Ichthyosaurus; and thirdly, and most importantly, that even if this had been the way in which the limbs had been differentiated, it would not be at all inconsistent with the possession of an innate power of producing, and an innate tendency to produce similar and symmetrical homological resemblances. It would not be so because resemblances of the kind are found to exist, which, on the Darwinian theory, must be subsequent and secondary, not primitive and ancestral. Thus we find in an

OF AN ICH

ANTERIOR EXTR

POSTERIOR EXTRE

les of the Secondary period, with long necks, small heads, and paddle-like limbs-are of yet higher organization than are the efts and other Amphibia. Nevertheless they present us with a similarity of structure between the fore and hind limb, which is so great as almost to be identity. But the Amphibia and Plesiosauria, though not themselves primitive vertebrate types, may be thought by some to have derived their limb-structure by dir

OF A PLES

logous structure should also be developed.[183] In the allied form called the slow lemur (Nycticebus) we have certain arrangements of the muscles and tendons of the hand which reproduce in great measure t

ious that similar abnormalities are often found

?té." And he afterwards quotes from Weitbrecht,[187] who had "observé dans un cas l'absence simultanée aux deux mains et

SCLES AND TEND

mis digitorum. F.p. Flexor profundus d

h little fingers; also one case in which the right little finger and little toe were so affected; six in which it was both the little finge

semble the primary wing-feathers, and are totally unlike the fine down which naturally grows on the legs of some birds, such as grouse and owls. Hence it may be suspected that excess of food has first given redundancy to the plumage, and then that the law of homologous variation has led to the development of feathers on the legs, in a position corresponding with those on the wing, namely, on the outside of the tarsi and toes. I am strengthened in this belief by the following curious case of correlation, which for a long time seemed to me utterly inexplicable,-namely, that in pigeons of any breed, if the legs are feathered, the two outer toes are partially connected by skin. These two outer toes correspond with

ical tendency. My friend and colleague Mr. George G. Gascoyen, assistant surgeon at St. Mary's Hospit

teries in the usual manner. In the second case an aberrant artery was given off from the radial side of the brachial artery, again almost at its origin. This aberrant

a pattern more complex and irregular than the spots upon a map-there is not one spot or line on one side which is not represented, as exactly as it would be in a mirror, on the other. The likeness has more than daguerreotype exactness." He goes on to observe: "I need not describe many examples of such diseases. Any out-patients' room will furnish abundant instances of exact symmetry in the eruptions of eczema, lepra, and psoriasis; in the deformi

es, are often not only symmetrically, but similarly, affected with psoriasis. So are the elbows and the knees; and similar portions of the thighs and the arms may be found affected with ichthyosis. Sometimes also specimens of fatty and eart

ortion of the right hand is greater than that of the left, of the right foot greater than that of the left foot." In another (Elizabeth Alford) lepra affected the extensor surfaces of the thoracic and pelvic

relations, adding arguments in favour of antero-posterior homologies, which it is here unnecessary to discuss, enough having been said

each living organic whole as well as by each non-living crystalline mass, and that there is such internal power or tendency, which may be spoken of as a "polarity," seems to be demonstrated by the instances above given, which can easily be multiplied indefinitely. Mr. Herbert Spencer[195] (speaking of the reproduction, by budding, of a Begonia-leaf) recognizes a power of the kind. He says, "We have, therefore, no alternative but to say that the living particles composing one of these fragments have an innate tendency to arrange themselves into the shape of the organism to which they belong. We

," has a distinct chapter on the "Analogy between Symmetry and Polarity," il

cope should reveal peculiarities of structure corresponding to peculiarities of habitual tendency in the embryo, which at its first formation has no structure whatever;"[199] and he adds that "there is something quite inscrutable and mysterious" in the formation of a new individual from the germinal matter of the embryo. In another place[200] he says: "We know that in crystals, notwithstanding the variability of form within the limits of the same species, there are definite and very peculiar formative laws, which cannot possibly depend on anything like organic functions, because crystals have no such functions; and it ought not to surprise us if there are similar formative or morphological laws among organisms, which, like the formative laws of crystallization

his tendency is controlled and subordinated by the action of external conditions, and not that this symmetry is super

directing its development as a crystal is built up, only in an indefinitely more complex manner, it is congruous to imagi

wer by which a particle of structureless sarcode develops successively into an egg, a grub, a chrysalis, a butterfly, when all the c