are due to a specific organization of the germ rather than to specific stimuli.
Why does one egg give rise to a chicken and another to a duck, or a fish, or a frog? Why does one egg give rise to a black guinea-pig and another to a white one, though both may be produced by the same parents? Why does one child differ from another in the same family? Why does one cell give rise to a gland and another to a nerve, one to an egg and another to a sperm? If these differences are not due to environmental causes, and the evidence shows that they are not, they must be due to differences in the structures and functions of the cells concerned.
Many differences in the material substances of cells are visible, and many more are invisible though still demonstrable. These differences may not be detectable by chemical or physical tests, and yet they may be demonstrated physiologically and developmentally. The most delicate of all tests are physiological, as is shown by the Widal test in typhoid fever, the Wassermann reaction in syphilis, the reactions of immunized animals to different toxines, etc. Lillie has recently shown that egg cells give off a substance which he calls fertilizin, which can be detected only by the way in which spermatozoa react to it. No chemical or physical test can distinguish between the different eggs or spermatozoa produced by the same individual, but the reactions of these cells in development prove that they are different. Undoubtedly chemical and physical differences are here present, but no chemical methods at present available are sufficiently delicate to detect them. The developmental test proves that there must be as many kinds of germs as there are different kinds of individuals which come from germs. It is one of the marvelous facts of biology that every individual which has been produced sexually is unique, the first and last of its identical kind, and although some of these individual differences are due to varying environment, others are evidently due to germinal differences, so that we must conclude that every fertilized egg cell differs in some respects from every other one.
But are there molecules and atoms enough in a tiny germ cell, such as a spermatozoon, to allow for all these differences? Miescher has shown that a molecule of albumin with 40 carbon atoms may have as many as one billion stereoisomers, and in protoplasm there are many kinds of albumin and proteins, some with probably more than 700 carbon atoms. In such a complex substance as protoplasm the possible variations in molecular constitution must be well-nigh infinite, and it can not be objected on this ground that it is chemically and physically impossible to have as many varieties of germ cells as there are different kinds of individuals in the world.
Even with regard to morphological elements which may be seen with the microscope it can be shown that an enormous number of permuta-
