« 이전계속 »
WHAT IS HEREDITY?
The development of an animal, 'with the complex and beautiful structural adjustments, the instincts, habits, and individual traits of its parents is one of the most wonderful phenomena of the material universe—Heredity is not due to the external conditions which act upon the ovum, but to something within the ovum itself—The phenomena of reversion—Asexual and sexual heredity—Possibility of an explanation of heredity— Characteristics which are now hereditary were at one time new variations—Heredity and variation are opposite aspects of the Bame problem—We may hope that a more perfect acquaintance with the laws of heredity will remove many objections to the theory of natural selection.
To the ordinary unscientific reader the word heredity may perhaps suggest nothing more than a few curious cases where an odd peculiarity of the parent has been transmitted to the children, or it may recall the hereditary transmission of a tendency to certain diseases, or the mental or moral idiosyncrasies of the parents.
To the breeder of domestic animals or plants it has a somewhat wider significance, and recalls the transmission by choice or fancy breeds of the features which give them their value. To him heredity is the law which enables him to modify his animals and to build up and perpetuate new varieties.
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 U3 that greater knowledge would show us, in the structure of the dog, an explanation of all that fits the dog for hia 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 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 is 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.