2o6 POPULAR SCIENCE MONTHLY. 



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 environ- 

 ment 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 inter- 

 action of these two factors, heredity and variability, shows that a con- 

 tinual 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 ani- 

 mal 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 physiologj^ was yet a book sealed with seven seals. Dur- 

 ing 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 phe- 

 nomena 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 unex- 

 pected uniformity. 



The labors of such men as Hertwig, Boveri, van Beneden, Stras- 

 burger, 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 observa- 

 tion 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 cre- 

 dentials 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 Yries has been busy making 



