246 
THEORIES OF HEREDITY. 
Nov., 1889. 
part they play in the economy of the organism. Tims, in 
man, the upper skin, or epidermis, is composed of layers of 
cells, becoming horny scales on the surface. These cells are 
continuous with those lining the digestive tract and which 
pass up the ducts into the various glandular organs. The 
connective tissues which bind the various structures together 
and which make up many parts, such as tendons and the 
lower skin or dermis, are also composed of cells and fibrous 
elements derived from cells. The supporting tissues, bone 
and cartilage, are also composed of cells and structures 
derived from cells ; and the same is true of the great contrac¬ 
tile tissues, striped and unstriped muscular fibre, and of the 
elements of the nervous system—nerve-cells and nerve-fibres. 
Out of many of these elements the complex organs are built 
up, with the addition of peculiar or specific cells of their 
own. 
All the varied units which compose the metazoan body 
may be classified under two chief heads. There are the cells 
which are concerned with maintaining the life of the 
individual—the body-cells or somatic cells ; and there are those 
which are concerned with maintaining the life of the species 
—the reproductive cells or yerm-cells. 
In the higher animals, the latter are aggregated in a com¬ 
paratively limited area, the reproductive organs (ovaries or 
testes). These can be removed in the operation of castration 
without essentially affecting the somatic cells. The life of 
the individual continues to its normal length under these 
circumstances, but the succession of individuals is entirely 
prevented. 
The problem of heredity may be stated as follows :—How 
is it that a single germ-cell can produce, by repeated division, 
an organism in which the peculiarities of the somatic units of 
the parent are reproduced? A single cell separates from a 
small area in the body of the parent, but it controls the 
development of the offspring, so that the characters of every 
part of the parent are repeated with more or less accuracy. 
It seems that there are only two possible ways in which 
this marvellous fact can be explained. First, the whole of 
the somatic cells may be so intimately connected with the 
germ-cells that each of the latter bears within itself the 
influence of the whole of the former—an influence, too, of 
such a nature as to lead to the reappearance of the corre¬ 
sponding somatic cell in the course of development; clearly, 
therefore, an influence of a material nature. Secondly, we 
may look upon the germ-cells as directly developed from the 
germ-cell from which the parent arose. Parent and offspring 
