Page:Popular Science Monthly Volume 77.djvu/30

24 of this possibility, we are not as yet entitled to explain all blending away as illusory; but we may bear in mind that this may be the case. It can at least be said, that scarcely a month passes without some case of inheritance, formerly seeming inscrutable, being brought into the field of well-ascertained law.

With the incoming of the idea of unit characters, passes our former conception of continuous variability. Supposing every character to be at all times variable—that is in motion, as it were, away from its present center of stability—there is no doubt that continuous selection would be required to keep characters up to any particular standard. The extraordinary permanency of some organic characters should suffice to make us doubt this necessity. For millions of years, certain features in the lower animals have been handed down generation after generation, practically without change. When we remember the tremendous complexity of the protoplasm molecule and the much greater complexity of the least imaginable bearer of heredity, and the fact that it has not been possible to break up and then reform the combination, as in inorganic chemistry, the permanency of these units in time is simply amazing. Least particles of protoplasmic jelly, they have stood while the rocks have been ground to dust, and made over many times. They are entitled to be ranked among the most permanent things in nature.

What then of the facts of variability, as they appear to us? What is the use of denying continuous variability, in the face of the fact that no two human beings are alike? The paradox may be resolved, when we remember the extraordinary number of words in the English language, no two the same—yet made up of the undeniably unchanging letters of the alphabet. When we recall that, on the unit character theory, the units in man must be exceedingly numerous, and must be recombined in almost every conceivable way in bisexual inheritance, it is easy to see that the chances against any two individuals coming out exactly the same are so great that such a result is practically impossible. The only case which can come under this head are those of identical twins, where the resemblance is indeed amazing, throwing light on the extraordinary potency of inheritance. Such twins are believed to result from the division of a single fertilized ovum, and hence to be, in a biological sense, two halves of a single individual.

Much light has been thrown on the permanence of unit-characters by studies among plants and protozoan animals of what are called pure lines. A pure line is one in which all the individuals have the same ancestry, uncontaminated by crossing. The most remarkable results have been obtained by Professor Jennings in his studies of Paramecium.

He says:

In a given "pure line" (progeny of a single individual) all detectible variations are due to growth and environmental action, and are not inherited.