It was a main objection against the doctrine of Lemery, that if two germs came in contact by accident (as he supposed), they could not exhibit any regularity in their mode of attach ment, but faces would be forced into chests, abdomens into spines, and so on. The mo derns, who adopt the same hypothesis, sup pose that the ova come in contact not by ac cident, but by an attraction de soi pour soi, of which the influence is, that the two ova beimg by accident set face to face or back to back, or in any other way similia simians, will be drawn to each other, and will unite by similar parts. But, with all respect for the authority of M. Geoffroy St. Hilaire and his disciples (as Mr. Paget elegantly said in his abstract of my monograph upon double monsters), who regard this as " la regle supreme de tous les arrangements et de toutes les mo difications organiques chez les etres com poses," I confess that I can find no good evidence that such an attraction exists. I can see in it nothing more than a very happy expression of a fact, which it in no wise ex plains. The extraordinary notion of MM. Delpech and Caste, that such an attraction may be the result of electric currents, is certainly no evidence of its existence, this being entirely imaginary. And the reasoning in its favour seems no better than the facts, for I can find nothing but this kind of circle : monsters adhere by similar parts, therefore there is an attraction de soi pour soi; there is then such an attraction, and therefore double monsters so adhere. I believe, therefore, that such an attraction is hypothetical ; and if it be so, surely the hypothesis which involves it and an accident as essential elements is less probable, as well as less sufficient, than that which I maintain. It is scarcely better than Lemery's, of mere accident; for it requires not only the accident of a particular position of the ova, but that of their being of the same sex, which has been said previously to be the general rule for double monsters. After all this, we conclude with the words of the learned Wolff, published in 1773 : Patet, igitur, monstra composita non sic oriri, ut aliquando duo scparati integri embryones fue rint, qui dein contingentes et compressi, par tibus eorum nonnullis dcstructis, aliis coalitis ct commixtis, concrescerent in unum novum compositum corpus; ea vero, qum vet defectu partimn vet insolita structura monstra sunt, non ita fieri ut prius integri et naturales em bryones fuerint, qui deinde per causas acci dentales ad gencrationem non pertinentes mutilati vet transmutati fuerint ; sect necesse esse, ut utraque monstrorum genera a primis suis initiis jam ejusmodi monstra fuerint." If we are right in not admitting the exist ence and the fusion of two distinct germs, we necessarily adopt the opinion that only one germ has been formed, and that in this excess of formative power lies the cause and origin of every monstrous duplicity. Mr. Allan Thompson demonstrated, in coincidence with Wolff, Von Baer, and Reichert, as may be seen in the wood-cut (fig. 629.), that upon one yolk, and in one germinal mem brane or blastodermatic vesicle, there may he formed, in birds, two primitive grooves, which, in their ulterior increment, shall probably form a double monster, as may be seen in a goose's egg, after five days' incubation, represented after Allan Thompson, infig.630. By the for mation of such a double primitive groove in a single ovum, we may explain the origin of the principal types of double monsters ; and on this point a recent observation of Valentin seems particularly worthy of notice, viz. that
in which an injury, inflicted on the caudal ex tremity of an embryo on the second day, was found, on the fifth, to have produced the rudiments of a double pelvis and four in ferior extremities. But if we admit this cause for those large and principal types, we must acknowledge that it is insufficient for the heteradelphs, and for all those cases in which, the body remaining single, some parts are double. For these the excess of formative power is the only explanation we can give. We understand, under this name, not the nisus fornzativus of the ancient physiologists, working as a Deus ex maehina, but the physical and vital metamorphoses nzateria., to which the formation of a new being ought to be attri buted. Those who are fond of the modern nomenclature may name it, if it pleases them, typical or organic power. But enough, we admit such a power, of whatever name it may he, and contend further, that different degrees or quantities of excess lie at the origin of all the cases of double monsters, the degree of excess determining in each the degree of duplicity. If a certain excess of power be admitted capable of producing any one case of duplicity, other amounts of excess may be believed capable of producing all the other cases which differ from it only in degree ; and I meant to have proved, in my monograph, and in the succinct survey I have given of double mon sters in this Article, that all double monsters may be referred to differences in degree of deviation from the normal singleness.
But in many double monsters we see excess in one part and defect in another, so that we must suppose, in our hypothesis, that in these cases the power, more or less excessive in quantity, is also wrongly distributed. Nor is this inconceivable; for since, in the normal developemental power, we must imagine at least two elements, quantity and distribution, and must acknowledge that, for the attain ment of a perfect result, the quantity must be distributed in definite proportions to each part, so it is not improbable that, in certain circumstances of fault in the ovum, a normal or an excessive power may be distributed dis proportionately in the several parts.
To sum up, therefore, our reasons for re jecting the hypothesis of fusion of ova in favour of that of excess or irregular distri bution of developemental power, for preferring to regard them as examples rather of single ness tending to duplicity than of duplicity tending to singleness, are briefly these : that it is probable that the whole class of monsters by excess owe their origin to different de grees of one common fault, and consequently that the explanation of their origin ought to be the same for all ; that no kind of fusion can account for the production of super numerary individual organs, the rest of the body being single ; but that it is not impos sible that excess of power in the ovum, which all admit can alone explain the lower degrees of duplicity, may, in proportionally higher degrees, perhaps by the formation of two primitive grooves, produce the more complete double monsters, or even two such separate individuals as are sometimes found within a single amnion.