ARTIFICIAL AND NATURAL SELECTION 35 



specific weight, and selecting from among thousands of beets those 

 which sank. The second was the determination of the specific weight 

 of the sap expressed from the tissues. It prepared the way for the 

 polarization. This principle was introduced about the year 1874 

 in Germany. It allowed the amount of sugar to be measured directly 

 and with very slight trouble. Thousands of beets could yearly be 

 tested, and the best chosen for the production of seed. The technical 

 side of these determinations has since been steadily and rapidly 

 improved. In some factories the exact determination of three hun- 

 dred thousand polarization-values is effected within a few weeks. 



It would take me too long to go into further details, or to describe 

 the simultaneous changes that have been applied to the culture of 

 the elite. The detailed features suffice to show that the chief care 

 of the breeder in this case is a continuous amelioration of the 

 method of selecting. To these great technical improvements it is 

 manifest that the progress of the race is in the main due, and 

 not solely to the repetition of the selection. 



Similar facts may be seen on all the great lines of industrial selec- 

 tion. And whenever the method has reached its height, the race is 

 soon surpassed by another, started from another varietal choice, or 

 selected according to a better principle. 



Applying this experience to the processes which are assumed to 

 occur in nature, we may obviously assert that only in cases of a con- 

 tinuous change of the life-conditions in one and the same direction 

 any real improvement of races may be expected. All other cases will 

 only be capable of yielding local races, and such, no doubt, are very 

 numerous. Selection keeps up the qualities some degrees above the 

 standard of the species, and it must do so for one strain in one 

 sense and for others in diverging directions. These local races, how- 

 ever, will always remain dependent upon their specific life-conditions, 

 and never become constant in this sense of the word, that the assumed 

 qualities should become independent specific marks. Even continued 

 environmental changes do not seem to be adequate to produce 

 lasting improvements. 



Until now we have simply contrasted the mutations and the 

 fluctuations. I have tried to show that both of them are subjected 

 to selection, as well artificial as natural. In nature, and in the long 

 run in practice too, the selection of the products of mutations plays 

 by far the largest part. Selection of the products of fluctuating vari- 

 ability gives rise to inconstant, and therefore only temporary races. 

 In practice the process is called improvement, in nature it produces 

 the local adaptations and local races. It is not known to yield any 

 permanent and independent results, the real results remaining 

 always dependent on the permanency of the agency of the selective 

 process itself. Thus interspecific selection is the broad base of pro- 



