VARIATION AND HEREDITY 33 



is permanent, so that even under normal cultural conditions a reversal 

 never takes place. The variety, thus produced, will hence remain constant 

 in a natural environment, although there always remains the possibility that 

 by the action of other agencies a return to the original condition may 

 be induced. Accidental reversions are as is well known, by no means 

 uncommon in the higher plants. 



It is also possible to induce in certain bacteria variations resembling 

 those mentioned above, but which are not permanent and gradually dis- 

 appear under normal conditions, though perhaps not until thousands of 

 successive generations have been produced. Since in the case of higher 

 plants this would require thousands of years, no perceptible reversal would be 

 noticed by observations lasting only during the lifetime of a single observer. 

 Hence, in considering this, and similar questions, the rapidly growing, 

 rapidly living, and rapidly reproducing lower forms are of the utmost value. 



External conditions act not so much as direct formative, as indirect 

 inducing agents, and thus produce vital changes, leading to an attain- 

 ment of new hereditary peculiarities. It is easy to understand why the 

 production of variations of this character should be favoured by subjecting 

 the organism to unusual conditions or requirements, for the disturbances 

 and the unusually labile condition thereby induced may originate changes 

 which reach or even exceed the limit of physiological elasticity, and hence 

 lead ultimately to a permanent modification of the original internal 

 constellation, that is to say, to a variation capable of hereditary trans- 

 mission. But this takes place only in special cases ; all modifications 

 induced by external conditions do not attain hereditary value, but for 

 the most part come and go with varying environment, either in the same 

 plant or from generation to generation. 



As concrete examples illustrating these points we may take either 

 a steel spring, in which permanent alteration is only produced when it 

 is bent beyond its limit of elasticity, or a musical-box, the melodious 

 harmony of which is permanently modified when one or more of the 

 pins of the cylinder are bent or broken. Such a change might appear 

 suddenly, and apparently without cause, nor need the same external 

 agency necessarily always produce a similar result. At the same time, it is 

 easy to understand that changes, or conditions preparatory to change, might 

 be induced in the internal arrangement or constellation by continuous use, 

 causing wear and tear, while these changes might gradually or suddenly 

 become manifest by alterations in the musical character of the melody. 



Saltatory variations often do appear in organisms, and may arise 

 under precisely similar external conditions in particular individuals only, 

 or may even affect these in different manner or degree. Bacteria afford, 

 however, very good examples of graduated variations affecting all the indi- 

 viduals to a similar extent. Thus, by certain methods, the production of 



PFEFFER D 



