To the naturalist, on the contrary, the word is filled with deep meaning, and instead of recalling to his mind a few odd cases, the tricks and accidents of heredity, it brings before him the most marvellous of all the phenomena of the material universe: the production from a simple egg of a living animal, with the intricate structure and complex bodily and mental functions of its proper species.

Thoughtful men of all ages have regarded the structure and faculties of the higher animals as a proper field for life-long study. Yet the acute intellects, the powers of patient observation and profound reflection which generations of naturalists have brought to this fascinating subject, have not yet given us a complete knowledge of the life of a single animal.

In every age and country where science has flourished men have devoted their lives to this subject, and have felt that their hardly-earned results could scarcely be called a beginning. So vast is the field, so many are the phenomena, that the province of natural science is practically infinite, for each animal and each plant presents special problems which open out in all directions before the student in an endless vista.

Wonderful and various as the attributes of each animal are, however, they are not mysterious; for, at the same time that we discover in an organism the power to do wonderful things, we also find in it a material organization, a mechanism, adopted to do these very things. It is true that we cannot perfectly understand this mechanism, that in many cases we fail completely in our attempts to trace its working, and that in most cases our insight is very crude indeed. Still we are able to show that the machinery exists; and anatomy, or the study of structure, goes hand in hand with the study of the bodily

and mental activities of animals. We do not understand the machinery, but we find that it is there, and we can interrupt its work by obstructing or injuring it. Our wonder is not a feeling of mystery, a sense that the phenomena transcend knowledge; it is due to a perception of the amount of knowledge required. We regard an adult animal with feelings similar to those with which an intelligent savage might regard a telephone or a steamboat.

A dog, with all the powers and faculties which enable him to fill his place as man's companion, is a wonder almost beyond our powers of expression; but we find in his body the machinery of muscles and veins, digestive, respiratory, and circulatory organs, eyes, ears, etc., which adapts him to his place; and study has taught us enough about the action of this machinery to assure us that greater knowledge would show us, in the structure of the dog, an explanation of all that fits the dog for his life; an explanation as satisfactory as that which a savage might reach, in the case of the steamboat, by studying its anatomy.

Let our savage find, however, while studying an iron steamboat that small masses of iron, without structure, so far as the means at his command allow him to examine and decide, are from time to time broken off and thrown overboard, and that each of these contains in itself the power to build up all the machinery and appliances of a perfect steamboat. The wonderful thing now is not the adaptation of wonderful machinery to produce wonderful results, but the production of wonderful results without any discoverable mechanism; and this is, in outline, the problem which is brought before the mind of the naturalist by the word heredity.

Every one knows that each dog exists at some time

is

as an egg, and the microscope shows in this egg no traces of the organs of the dog's body or of anything at all like them. So far as our means of examination go the egg no more like a dog than the mass of iron is like a steamboat. It may be said, though, that the dog's egg is not left to itself, but is fertilized and is carried inside the body of the mother until the new animal is matured; that it is there nourished and built up from substances supplied through the body of a full-grown dog; that it may be acted upon at this time by agencies which have a direct tendency to build up out of it an organism like the parent; that the egg does not actually contain a potential dog, but simply supplies the proper material to be acted upon by the surrounding conditions, and that the structure of the new animal is due to these conditions; that the embryo becomes a dog because it is bathed by a dog's blood, nourished through a dog's body, and is completely surrounded by influences which are peculiar to dog nature. Those persons who are not naturalists derive their knowledge of the animal world chiefly from our common domestic animals, and to such persons this explanation may seem probable; but naturalists, with wider experience, know that animals which carry their young inside their bodies are exceptions, and that the organization of the future animal must exist potentially in the egg, since the conditions to which it is exposed cannot possibly have any tendency to produce from it a being which does not already exist, in some form, within it.

A bee is almost as wonderful as a dog; its anatomical structure is exquisitely delicate and complex, and every one is acquainted with the wonderful work which it accomplishes. At the time it is laid the egg which is to become a worker bee contains no visible trace of its

body, or of anything like it. It has been carefully studied with all the resources of modern science, but examination shows nothing within it which is more like a bee than a mass of iron is like an iron ship. This egg is not even fertilized, but it develops into a perfect worker, with all its wonderful structure and instincts, by virtue of something which it contained when it left the ovary of its mother. It is true that it is not left quite to itself, but is carefully attended and cared for by other bees; but everything which they do for it might be done just as well by delicate machinery, and the attention has no tendency whatever to manufacture a bee. Proper heat and access to air are as necessary as attention, and attention has no more power to produce a bee than air or heat.

No one who is familiar with marine animals can believe for an instant that the conditions to which an egg is exposed have anything whatever to do with the character of the animal to which it gives rise. We may artificially remove eggs from the ovaries of several different animals, fertilize them artificially, and then place them together in a tumbler of sea water, and expose them to exactly similar external conditions, yet each one will follow its own determined course, and we may rear in the same tumbler of water from eggs which are hardly distinguishable animals which have less in common than a dog and a bird.

If there is no mystery in the performance by the complicated organs of an adult animal of all its complicated functions, what shall we say when we find the power to perform these functions existing in a latent state in the egg, without the corresponding organs?

This is the problem of heredity. In the mind of the naturalist the word calls up the greatest of all the wonders of the material universe: the existence, in a simple,

unorganized egg, of a power to produce a definite adult animal, with all its characteristics, even down to the slightest accidental peculiarity of its parents; a power to reproduce in it all their habits and instincts, and even the slightest trick of speech or action.

This is by no means the whole of the problem of heredity. One of the most interesting phenomena connected with our subject is what is known as reversion, or the appearance in the child of peculiarities which were not present in either parent, but are due to inheritance from a grandparent or a more remote ancestor. An interesting illustration of this law is the occasional appearance in horses of stripes on the body and legs. Such stripes are not usually present in the horse, although Darwin has given reasons for believing that our horses are descended from a striped zebra-like ancestor. The power to revert to this ancestral form is handed down from generation to generation in the egg, and it may show itself at any time by the production of a striped colt. Reversion is, in a certain sense, exceptional, but it is not at all rare, and we must add this power to the wonderful properties of the egg.

Darwin gives the following case, which will serve to illustrate the nature of reversion: A pointer bitch produced some puppies; four were marked with blue and white, which is so unusual a color in pointers that she was thought to have played false with one of the greyhounds, and the whole litter was condemned, but the gamekeeper was permitted to save one as a curiosity. Two years afterwards a friend of the owner saw the young dog, and declared that he was the image of his old pointer bitch, Sappho, the only blue and white pointer of pure descent which he had ever seen. This led to close inquiry, and it was proved that he was the great