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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 i» 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 13 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. Keversion 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. -, . .
great-grandson of Sappho; so that, according to the common expression, he had only one-sixteenth of her blood in his veins.
Another aspect of our subject must be kept constantly in mind. Among the higher animals heredity usually manifests itself only by what is known as sexual reproduction,—that is, the development of new individuals from fertilized eggs; but in the lower forms of life another kind of reproduction, the development of new individuals by budding or by analogous processes, is even more common. Among the hydroids heredity may manifest itself by the formation of new animals, with all the characteristics of the parent, on almost any part of the body of the latter, and in certain plants the smallest fragment of tissue may become a new and perfect plant", capable of producing others in the same way or by seeds. The most sure and rapid way to get new sea-anemones is to tear an old one to pieces. As a rule this power is confined to the lower forms of life, but certain animals which are by no means low or simple in structure multiply asexually, and the offspring thus produced inherit, like those developed.from eggs, all the characteristics of the parent.
This then is the problem of heredity, certainly one of the grandest secrets of nature. When we reflect upon its obscurity and complexity we may fairly ask what hope there is of discovering its solution; of reaching its true meaning, its hidden laws and causes. If it is trne that, in each egg, all the functions and faculties of a definite mature animal lie hidden, without any corresponding organs, must we not regard heredity as a mystery too great for solution; as something which must be accepted as it is without scientific explanation?
Thirty years ago the adaptation of each organ of an adult animal to its proper purpose seemed to be a mystery of the same kind, and many profound thinkers satisfied themselves and taught others that this adaptation was not brought about by the laws of matter and by secondary causes; that it must be accepted in itself, without explanation, and that the methods of physical science are here of no use.
Darwin's work has taught us that this is not true; that in the law of natural selection we have at least a partial explanation of the origin of the adaptation of nature; that while natural selection may not be the exclusive means by which they have been produced, it is, so far as it goes, a true scientific explanation, for it even puts it in our power to produce, in domestic animals, similar adaptations to special purposes, by the selection of the fittest variations.
Darwin, in his first and in all his later books on the subject, pointed out that his discovery did not complete the solution of the problem; that " natural selection is a great but not the exclusive means of modification." The greatest value of his work lies in the proof which he has furnished, that the origin of the structure of animals is not beyond our reach, but that observation and reflection, the means which have unlocked for us so many of the secrets of inorganic nature, are equally useful in this field; that the adaptations of nature may be studied and understood like a problem in astronomy or physics.
The aim of this work is to show that the same thing is true of the problem of heredity.
We may not be able, as yet, to penetrate its secrets to their inmost depths, but I hope to show that observation and reflection do enable us to discover some of the laws upon which heredity depends, and do furnish us with at least a partial solution of the problem; that we have every reason to hope that in time its hidden causes will all be made clear, and that its only mystery is that which it shares with all the phenomena of the universe.
In this introductory statement we have presented one side of the problem of heredity: the transmission from parent to child of the established congenital hereditary characteristics of the race. We must not forget, though, that there is another aspect which is fully equal to this in importance. We know that each characteristic has been gradually acquired through a long series of modifications; that all the wonderful adaptations which fit animals to their surroundings, and meet their particular needs, has been evolved step by step by the natural selection of the fittest congenital variations. Each racecharacteristic has at one time been a new variation, and the process of modification is still going on and perfecting the harmony between the structure of each organism and its needs. No theory of heredity has any value unless it explains the way in which new features, which may become hereditary, continually make their appearance as congenital variations, at the same time that it accounts for the way in which established peculiarities are handed down from generation to generation.
The. problem is two-sided; what is now hereditary was at one time variation, and each new variation may soon be hereditary. Heredity and variation are opposite aspects of the same thing, and an explanation must be examined and tested on the one side, as well as on the other, before it can be accepted.
There is still another consideration which remains to be noticed.
Darwin has never failed to perceive, and he has frequently pointed out,that the law of natural selection is not