Page:Popular Science Monthly Volume 85.djvu/239

Rh denominations. If these cards are then dealt into two hands, one card of each pair going to one hand and the other to the other hand, we will have two cards of each denomination in each hand, but if the cards are dealt indiscriminately some of them will be red and some black. This description parallels what takes place in the maturation of the human ovum, except that there is no evidence that there are more than two suits of chromosomes, one maternal and the other paternal.

To carry out this comparison in the case of the maturation of the human sperm where there are only 47 chromosomes it is necessary to take another pack and discard an additional card, say the queen of clubs; then in the union of corresponding red and black cards into pairs the queen of hearts unites with the queen of spades, but the queen of diamonds remains alone, and when the cards are dealt into two hands as before one hand will contain 24 cards and the other 23.

If now we complete this comparison by extending it to what takes place in fertilization we must take one hand from each of these deals and put them together into one pack; though this pack would contain cards of every denomination there would be varying numbers of red and black cards and a mixture of cards from two distinct packs. In no game of cards do corresponding cards from different packs have slightly different values nor are half of the cards taken from one pack and half from another (at every game), but this is just what happens in the shuffle and deal of the chromosomes. Because of the mixture of chromosomes from distinct individuals in every generation, each of which has its own peculiar value, the game of heredity becomes vastly more complex than any game of cards.

This illustration may serve to make plain the fact that the purpose of maturation and fertilization is, in part, this shuffle and deal of the chromosomes, and its result is that every oosperm and every individual which develops from it is different from every other one.

This conception of the specificity of every germ cell, as well as of every developed individual, sets the whole problem of heredity and development in a clear light. The visible peculiarities of an adult become invisible as development is traced back to the germ, but they do not wholly cease to exist. Similarly, the multidinous complexities of an adult fade out of view as development is traced to its earliest stages, but it is probable that they are not wholly lost. In short, the specificity of the germ applies not merely to those things in which it differs from other germs, but also to characters in which it resembles others—in short, to hereditary resemblances no less than to hereditary differences.

The mistake of preformation was in supposing that germinal parts were of the same kind as adult parts; the mistake of epigenesis was in maintaining the lack of specific parts in the germ. The development of every animal and plant consists in the transformation of the specific characters of the germ into those of the adult, but not in the formation