In order to determine the dormyn of inheritance in the causation of disease, it is necessary first to consider the nature of its influence in the normal anatomical and physiological conditions of our bodies. Looking broadly at the subject it may be said that the whole bodily conformation of the animal is dependent on inheritance. It is by inheritance that human beings belong to the Vertebrata, to the Mammalia, and to the genus and species Homo. It is also by inheritance that the more minute peculiarities of races, families, and individuals are produced. This matter is illustrated and elucidated in the observations which follow.

Race The Result Of Inheritance

We see the influence of inheritance not only in the fact that human parents produce human offspring, but also in that the offspring conform to the race of the parents. It is important in connection with what follows to consider what constitute the differences of the races of men. There is more or less pigment in the deeper layers of the epidermis; the hair, in transverse section, is circular or oval in outline, or more or less oval; the nasal bones are articulated at different angles, the lower jaw is more or less massive; the shape and details of the bones of the skull are various; the eyes are horizontal or are directed upwards at their outer extremities; and so on. It is these differences in structure that make the chief distinctions of race, and, if we compare two such dissimilar races as the negro and the Englishman, we shall see that the difference lies in a multitude of details, each of which if taken apart might seem trivial. And yet the fertilized ovum inherits the power of modelling the bodily frame down to these minute details.

Individual Characters The Result Of Inheritance

But, besides the characters which distinguish men as men, and the men of one race from those of another, we know that men are distinguished from each other individually. When we consider the general likeness of the men of one race to each other, it seems marvellous that, among the multitudes with whom we come in contact, we rarely mistake one for another. The only explanation of this is that there are innumerable shades of difference - in colour of skin, hair, and eyes; in shape of nose, mouth, and eyebrows - an infinite number of small peculiarities, which make up the whole portrait. We unconsciously blend the whole of them in our mental picture of the individual, and if one of them be altered we at once recognize that our portrait is out in some way, and has to be readjusted, as when we change the arrangement or length of the hair or beard, or when the mouth is altered by the loss of teeth or the addition of an artificial set. This may be further illustrated by the well-known fact of the insufficiency of photographs. In them we have an exact picture but without the colour, and the absence of this one element in the portrait has often the effect of almost destroying the likeness.

It need hardly be said that most of these fine shades of difference between man and man are due to inheritance. The ovum enters on its career with these already inherent in it, and the various minute points of difference are but repetitions variously compounded of points in its progenitors. We are so accustomed to look for resemblances between children and parents that we are apt to forget what these imply. It is a particular colour of the iris, a special tone in the voice, a trick of manner, such as a twitching of the upper eyelid, or a way of fidgeting with the hand, which recalls the parent or grandparent. These minute points of resemblance become the more remarkable when we remember that they are all inherent in the fertilized ovum. Without any external assistance it goes on modelling the tissues and endowing them with their functions according to a prearranged pattern, and this power continues throughout life; so that it often happens that, on to middle life or old age, points of resemblance come out which had not been previously visible. It is as if the mother and father transmitted to the ovum certain forces inherent in themselves and derived from their ancestors. These forces will be variously proportioned in each case, and the product will be a very complex one. It is quite impossible to tell how, in any particular case, these forces are mingled, but we can often identify the individual items as coming from this or that parent.

A highly important illustration of the truth of what has been adduced is afforded by the facts connected with Twins. We have seen that each ovum which leaves the ovary and becomes fertilized is thereby endowed with the forces which mould it into the future human being. These forces seem variously distributed in the ova and the spermatozoa, so that each ovum has them in such varying proportions that even the brothers and sisters of the same family are often very different. But if a single fertilized ovum produces two embryoes, then we might expect that the forces inherent in these two embryoes would be much more nearly alike than usual, and that in consequence the two individuals would resemble each other more closely than is usually the case. And so it is. There are two kinds of twins. In the one the individuals are derived from separate ova, just as are the progeny of animals which bear several young at the same time; while in the other kind, or homogeneous twins, there is only one ovum, which probably produces the twins by fission. (See further on, p. 3(5.) In the case of twins derived from the same ovum we may look for close resemblance, and they will always be of the same sex. In the other case, where they are derived from separate ova, they may or may not be of the same sex, and they will not necessarily resemble each other more closely than the ordinary brothers and sisters of a family. In a well-known work by Francis Galton we have a collection of interesting facts in illustration of the life-history of twins. He gives many anecdotes supplied by twins as to the mistakes made on account of close likeness, from which it will be gathered that the "Comedy of Errors" is scarcely a burlesque. The resemblance in many cases lasted throughout life, and extended to the smallest matters, even of feeling and thought, and, indeed, to their illnesses. From these facts we have demonstrative proof that in the fertilized ovum we have already inherent virtually all the finer details of structure and function, although many of these only come into effect in after-life. It will be observed that the sex is determined in the ovum.