10

CHAP. II.

ON HEREDITARY DISEASE.

THE subject of hereditary temperament and tendency to disease is still largely open to inquiry. Future research will unquestionably lead to an increased estimate of the influence of this cause on all the conditions of individual life, and not least on those of disease. Looking to analogy in plants and lower animals, we see in every direction the wide and various transmission of incidental varieties in each species; generally depending on combinations of natural causes; sometimes effected by human care and design. One general law, as respects man, seems to be that so ably developed by Dr. Prichard; viz. that all original or connate bodily peculiarities tend to become hereditary; while changes in the organic structure of the individual from external causes during life, commonly end with him, and have no obvious influence on his progeny.

*

I know not that this principle could have been in any way anticipated, or otherwise derived than from actual observation of facts. Nor is it certain that many exceptions do not exist to it; as well in the wide circle of structural changes from the effect of disease; as also more notably in those instances where necessities of situation alter certain parts of the organization of animals, and where the continuance of such change. is needful to their preservation under these new circumstances. The effects of domestication come under the latter view, where changes originally individual to the animal are

* Researches into the Physical History of Mankind.

propagated to its posterity; reaching gradually, as it would seem, a certain range of deviation for each, beyond which definite causes interfere to prevent a further change.* Nevertheless, as a general law, the principle just stated may be deemed highly probable, if not wholly proved; and it is one fruitful of important inferences, both in physiology and pathology. It comes in direct relation to the subject of hereditary diseases: and though the manner of their transmission is still a mystery, hidden in the same obscurity as the more general fact of the reproduction of the species, yet are we able to reason upon the effects, and to class them in certain relation to each other, and to the healthy and natural condition of the human frame.

It would be needless, and beyond my purpose, to cite the numerous examples of hereditary peculiarities of comformation, function, and tendency to disease. Such instances are familiar to us in common life, -in history and books,—and yet more especially and frequently in the practice of medicine. I have, indeed, been led to put together these remarks on the subject, from the numerous examples I found among my notes and recollections; to which I was desirous to give a more definite form, illustrative of their relations to each other, and to the general laws of animal life. As I have already remarked, we are still only partially acquainted with the number

* If other animals than those now possessed were needed for the uses or luxury of man, there is little doubt that human care alone might greatly extend the number of such varieties; and this in truth has been done, in many instances familiar to our knowledge.

There would seem to be cases, where certain combinations of muscles, or faculties of action, habitually called into exercise for particular effects, may produce bodily results more or less permanent, and capable of being conveyed to offspring. The pace of cantering in the horse, the motions of the tumbling pigeon, and the habits of certain varieties of the dog, may be noted in illustration of this view.

and singular variety of such cases. The greater exactness of modern observation is ever placing before us new and wonderful instances, in which the most minute peculiarities or defects, in structure and function, are transmitted from one generation to another. Scarcely is there an organ or texture in the body, which does not give its particular proof of these variations, so transmissible :- and we might almost doubt the permanence of the type of our species, thus largely and unceasingly infringed upon, were we not permitted to see something of those more general laws, by which the Creator has set limits to the change, and made even the deviations subservient to the welfare of the whole.

For in considering this hereditary tendency to disease, whether arising from structure or less obvious cause, it is needful to regard it in connexion with, or rather as part and effect of, that great general principle, through which varieties of species have been spread over the globe, with obvious marks of wise and beneficent design. That there are certain determinate laws by which the permanence of species is secured, must be admitted as a general fact, the result of all observations hitherto made on animal organic forms, both living and extinct. The very circumstance of the frequent repetition of analogous and parallel series of forms, under different physical conditions, and at different geological epochs, though admitting some doubt as to its interpretation, is nevertheless most favourable to this opinion.* The laws upon which these

* Still we are not entitled to deny to the followers of Geoffroy St. Hilaire the possibility that it may be otherwise; and that research may hereafter disclose to us some evidence that species are not immutable. If this were eventually proved to be so, it would in no wise affect the great argument of Natural Theology. A law presumed at one time to be universal, would be found collateral or subordinate to a still higher law; further removed, it may be, from our comprehension of its details, but involving the same proofs of design as the basis of the whole.

diversities depend, and by which they are limited, allow wide scope to their existence without injury or disorder of the general system. In the instances where such effects do occur, we can discern some part of the modifying causes, even amidst the profound obscurity which conceals from us their original source.

This subject, indeed, cannot be rightly pursued, without referring to those recent inquiries into the original types and development of animal life, from the simplest to the most complex forms, which form a new field within the great domain of human science. Such researches into what has been termed Embryology, though Hervey and Hunter prepared the way for them, have latterly engaged much less attention in England than among the naturalists of France and Germany.* The boldness, or even rashness, of some of the generalizations which have been attempted, particularly in the doctrine of fundamental unity of structure and design, must not prevent us from looking to a large increase of sound knowledge from these inquiries, based as they are upon a close and minute observation of facts. Their bearing upon the question of hereditary conformation and peculiarities, whether in families, tribes, or races of men, will be obvious to all who have studied this curious subject, and the controversies which have grown out of it. Dismissing the notion of a single primitive germ, or absolute unity of animal type, as a thing not proved, and

*In France, Geoffroy St. Hilaire, Lamarck, Blainville, Dumas, and Serres; in Germany, Meckel, Tiedemann, Baer, Spix, Valentin, &c., have been the most conspicuous of those lately engaged in the inquiry. In the former country, Cuvier sought to stay the extremes into which these doctrines have occasionally been carried. The sound and perspicuous understanding of this admirable man, while ever pursuing the highest generalizations to which facts would conduct him, had the power, no less important, to resist all inordinate speculation, and to recognise the bounds as well as the capacity of human research.

probably incapable of proof; and putting aside further some fanciful analogies which have defaced the inquiry, we yet must admit that comparative anatomy is ever carrying us back, by successive steps, to simpler and more fundamental types of animal life; - to conditions of structure, common to large and seemingly remote classes of created beings. It is still a subject of dispute where this progress upwards to more general forms is stopped by essential differences of type, or whether such divisions do at any point in the series really exist? —whether there are separate circles of analogies, as defined by the four great classes of Cuvier, or by the more complex systems of some later zoologists?-or whether these are but artificial limits, subordinate to a type pervading all the forms of animal life? These are questions strictly within the scope of research, and the solution of which connects itself with some of the highest objects of natural science.*

* Professor Whewell, in his History of the Inductive Sciences, and Dr. Roget, in his Bridgewater Treatise, have discussed these questions, as to general types and unity of design, in a happy spirit of philosophical inquiry. Some valuable papers on the same subject, by Dr. Martin Barry, will be found in the Edinburgh Philosophical Journal. Other eminent naturalists in our own country have now entered upon the investigation.

I have never, as already stated, seen cause to think the argument of Natural Theology at all affected by these inquiries; even when pushed to the speculations of a primitive germ, or original unity of type. Its truth indeed is far above the reach of what, after all, are but subordinate researches, even if they could by possibility attain the proofs of what these terms express. The chain is lengthened, and its parts are connected together by new and unexpected links. But still it is a chain of designed organization throughout; and if we simplify the first of these links, it is but to render more wonderful the number and perfection of the varieties which are evolved in definite forms from this elementary structure. If the parts in man have all their analogies or models in lower animals; or if, according to one view proposed, we regard the several organs of the human embryo as passing through all the gradations of lower types, before reaching their perfect development in man; still the great argument remains the same. The progressive elaboration throughout all its stages is definite