variability. Heredity, the conservative factor, by which what was once acquired is multiplied and rendered stable; variability, by which there appear side by side with those forms that are hereditarily constant, others which, being perhaps in yet closer harmony with the environment and with the prevailing conditions of life, may thus obtain a chance to defeat the first and to supplant them, with the prospect, however, of being in turn ousted by yet more closely adapted new forms, more exactly fitted to the surroundings. The mutual interaction of these two factors, heredity and variability, shows that a continual tendency in organic nature prevails, by which life proceeds from the simple to the more complicated, from the more primitive to the more perfect. And the archives in stone that have been opened to us by the geologists contain ample proofs to convince us that during the succession of thousands and thousands of centuries, plant life and animal life all over the world have passed through a similar process of development.
When Darwin was writing his 'Origin of Species' the chapter on 'Heredity' in physiology was yet a book sealed with seven seals. During the last forty) r ears, especially during the latter twenty, several most important pages of that book have been closely studied and partly deciphered. The conviction has begun to dawn upon us that the phenomena of heredity, assimilation and growth do not belong to different categories, and that, furthermore, this so mysterious heredity can be traced to the minute material particles with which it is bound up, and which through the whole range of plants and animals show an unexpected uniformity.
The labors of such men as Hertwig, Boveri, van Beneden, Strasburger, Guignard and many others, who have made a study of heredity, are especially important, because they have succeeded in analyzing the phenomena into their component elements, thanks to careful observation and experimental testing. They have shown us how very much we may yet expect from experimental work. In the next few years we may, no doubt, look forward to a rich harvest in this extensive field of investigation.
Variability is the name of the second group of facts, on which the slow development of higher, better and more complicated types of living beings rests. Here, too, we must call for the facts and ask for the credentials of the theories we meet. There is ample proof that in the domain of variability we encounter many delusive traps and a host of difficulties. Even Darwin has not spun out to the end the thread of comparative experimentation. It has been reserved to Hugo de Vries to point this out in a very remarkable book ('Die Mutationstheorie,' Leipzig, 1901-1903) that has just been completed.
For nearly twenty years Professor de Vries has been busy making