Page:Popular Science Monthly Volume 82.djvu/96

92 up of a flat layer of tissue on the yolk. Obviously in this case the gut did not exist as such in the germ.

It is unnecessary to multiply objections to this interesting bit of metaphysic. Both the epigenetic and the preformationist theories of the eighteenth century are dead and buried under the relentless logic of events. Essential forces and preformed miniatures, alike in their finality, were unable long to withhold the attention of naturalists from the more potent suggestions of a rapidly growing body of new observations.

With the discoveries that organisms are built up of morphologically equivalent protoplasmic units, or cells; that both egg and sperm are cells, also; that the nucleus, especially the chromatic substance, is the part of the cell chiefly if not wholly concerned with the inheritance of the individual and specific characters and their distribution in the developing organism; more than all, with the discovery of the essential nature of fertilization, new theories were devised to interpret the still puzzling problem of individual and specific differentiation. These, like their prototypes of the previous century, fall into two contrasting classes.

Both of these classes of theories recognize that individual differentiation can not be interpreted without regard to race development. The germ from which the individual springs has history behind it, is composed, indeed, of two fragments of two preexisting individuals, the parents, who, in turn, sprang similarly from a previous generation. It is at once apparent that all modern theories of development must reckon with these facts; which means that, however simple we may conceive a given germ to be, the probabilities are overwhelmingly opposed to the conception that it is homogeneous; and they are equally in favor of the conception that it possesses from the start, in view of its relation to a preexisting parent, some degree of differentiation.

In perfect accord with these requirements, modern epigenesis and modern preformation nevertheless exhibit characteristic differences. On the one hand, is the preformationist theory of determinants devised especially to explain the persistence, through many generations, of very trifling characters, such, for instance, as a small pit on a human ear, recognized as a family trait, or a spot on one surface of a butterfly's wing, or a lock of white hair on a particular area of an otherwise dark-haired head. Such characters appear to come and go without effecting in any way the other characters of the organism. This independent variability is interpreted on the assumption of fundamental living units in the chromatin of the germ nucleus that represent and determine all the various characters of every individual. The germ chromatin is accordingly conceived to contain the determinants of all the heritable characters; and these are further conceived to be so associated, that in the