more likely to cause death than to cause modification. The features of microcophalous idiots show us, however, that the shape of the skull and of the face is only due, in part, to heredity, and is, in part at least, due to the size and shape of the brain. In lop-eared rabbits the whole conformation of the skull is altered by the mechanical pressure of the drooping ears, and it is stated that certain monstrosities in the shape of snail-shells are due to the arrested development of the reproductive organs. Moquin-Tanden remarks that with plants the axis cannot become monstrous without in some way affecting the organs subsequently produced from it.

We can see, from the study of domesticated pigeons, that an increase or a decrease in certain organs is a direct cause of modification in other parts. Pouter pigeons have been selected for length of body, and the establishment of a long-bodied race has increased the number of their vertebræ and the breadth of their ribs. Tumblers have been selected for their small size, and the number of ribs and of primary wing-feathers has thus been reduced. Fantails have been selected for their large widely-expanded tails, with numerous tail-feathers, and the size and number of the caudal vertebræ have thus been increased, and the selection of long-beaked carriers has increased the length of their tongues. Cline states that the skull of a ram with horns weighs four times as much as that of a hornless ram of the same age, and Youatt states that in hornless cattle the frontal bones are materially diminished in breadth towards the poll, and the cavities between the bony plates are not so deep, nor do they extend beyond the frontals.

The kidneys of different birds differ much in size, and St. Ange believes that this is determined by the size of the pelvis.

It is plain that if the character of important parts can be thus changed by changes in other parts, the typical or characteristic form of these parts may be due only. partially to heredity.

We see then that the structural complexity of an adult animal is due in part to the formation of structures which are not alive, in part to the direct modifying influence of external conditions of life, and in part to the action of one organ of the body upon another, so that the number of features which are directly inherited is very much less than the number which are constant in and characteristic of the species.

It is impossible for us to state at present how many features must be subtracted from the race characteristics of an animal in order to give us the total number of hereditary congenital characteristics. The observations and experiments which are recorded are few in number, but they are sufficient to show us that, in all the higher animals, very considerable deduction must be made, and we may be sure that the mature animal is vastly more complex than the egg. There is still another limiting circumstance which has not yet been mentioned.

Many of the parts of an organism are due to indefinite multiplication of a single element. The simplest illustrations of this fact are the blood corpuscles of vertebrates and the leaves of plants. It is clearly unnecessary to suppose that each vertetrate ovum contains separate particles for all the blood corpuscles, or that each seed contains separate particles for all the leaves which the plant is to produce. All that is necessary is. to assume that it contains particles which are capable of producing a single one of these structures, with a capacity for indefinite multiplication, and that surrounding conditions determine how far, and in what places,

this power of multiplication shall manifest itself. Most of the organs of the body contain great numbers of cells which are alike both in structure and function, and as it is usually quite impossible to say how far the size of an organ is truly hereditary, and how far it is determined by surrounding conditions, it is, of course, impossible to say to what extent its mature structure is represented in the ovum, but as we know that the size of most organs varies, and may be increased or diminished by external influences, we may be quite certain that the number of independent cells which make up the tissues and organs of a mature organism, is very much greater than the number represented by distinct particles in the ovum.

It is not even necessary to suppose that all classes of cells which are present in the adult are represented in the ovum. In a mammal, for instance, certain epithelial cells become converted into hairs, while others become converted into glands or other specialized epithelial

structures.

It is not necessary to assume that all of these specializations are represented in the ovum, for we know that ordinary epithelial cells, in a part of the body where no hair is normally developed, may, when inflamed, give rise to hairs. It is therefore quite possible that each epithelial cell may, when excited by the proper influence, tend to become converted into a hair cell. Each cell of the body may possess the tendency to manifest certain properties under certain conditions, and to manifest certain other properties under other conditions, and the descendants of a single cell may thus become modified in several divergent directions, and each modification may be perfectly constant and characteristic of the race without being hereditary; that is, without being represented in the ovum by a particle with the same specialization.

礼

It may seem to some that the assumption that the egg contains particles capable of producing an unspecialized epithelial cell which shall have the power to give rise to all the specialized sorts of epithelial cells, involves just as much complexity of structure as the assumption that each kind of cell is represented in the ovum, but I think an illustration will show that this is not the case.

Training of a certain kind will develop a boy into a good pedestrian, while another sort of training will make him a good shoemaker; but it is surely simpler to assume that he is born with a tendency to develop the characteristics of a shoemaker under the influence of certain conditions, and those of a pedestrian under other conditions, than to assume that he is born with all the peculiarities of both latent in his organization.

The direct modifying influence of surrounding conditions is a subject upon which very much remains to be done, but we know enough about it already to state that many of the constant characteristics of organisms are due to exposure to constant and uniform conditions rather than to heredity. To what extent this is true we are quite unable to determine, but we can be sure that the organization of the ovum is simpler, and in all probability vastly simpler, than that of the developed organism.

After all these deductions are made the number of strictly hereditary features is very great indeed, and the egg of one of the higher animals must be a marvellous structure, for we know that, after all, most of the characteristics of an organism are not due to the influence of its conditions of life, but to the past history of the race; and Darwin has shown us that the successive changes which have resulted in the evolution of any organism do not, usually, owe their existence to the direct modify

ing effect of external influences, but to the natural selection of congenital variations.

The fact that our theory requires us to believe that the egg of one of the higher animals is complex beyond our powers of conception, must not be regarded as an argument against the theory, for we are compelled to believe this in any case. The difference between our theory and other attempts to explain the phenomena of heredity, is that it does what no other hypothesis attempts. It furnishes a simple explanation of the manner in which the ovum has acquired its present complexity.

In the following chapters I shall give some of the reasons for believing that the difference between the functions of the sexual elements which the theory requires does actually exist, but even in the absence of this proof it would be natural to conclude that if race modification could be furthered and aided by the divergent specialization of the functions of the two reproductive elements, natural selection would, in all probability, have acted so as to bring such a specialization about.

We know that the influence of natural selection is constantly exerted to seize upon and perpetuate any tendency to division of labor among the organs and tissues and cells of the body, and it is only natural that the successive stages in the specialization of the sexual elements should have been perpetuated like any other useful specialization.