LETTERS TO CARL NAGELI, 1866-1873 



never observed gradual transitions between the parental characters or a pro- 

 gressive approach toward one of them. The course of development consists 

 simply in this; that in each generation the two parental characteristics ap- 

 pear, separated and unchanged, and there is nothing to indicate that one of 

 them has either inherited or taken over anything from the other. For an 

 example, permit me to point to the packets, numbers 1035-1088, which I sent 

 you. All the seeds originated in the first generation of a hybrid in which 

 brown and white seed-coats were combined. Out of the brown seed of this 

 hybrid, some plants were obtained with seed-coats of a pure white color, with- 

 out any admixture of brown. I expect those to retain the same constancy of 

 character as found in the parental plant. 



The second point, on which I wish to elaborate briefly, contains the following 

 statement: "You should regard the numerical expressions as being only 

 empirical, because thev can not be proved rational." 



My experiments with single characters all lead to the same result: that from 

 the seeds of the hybrids, plants are obtained half of which in turn carry the 

 hybrid character {Aa), the other half, however, receive the parental charac- 

 ters A and a in equal amounts. Thus, on the average, among four plants two 

 have the hybrid character A a, one the parental character A, and the other 

 the parental character a. Therefore 2ylc+^+fl or ^-|-2.4a+ais the empirical 

 simple, developmental series for two differentiating characters. Likewise it was 

 shown in an empirical manner that, if two or three differentiating characters 

 are combined in the hybrid, the developmental series is a combination of two 

 or three simple series. Up to this point I don't believe I can be accused of 

 having left the realm of experimentation. If then I extend this combination of 

 simple series to any number of differences between the two parental plants, 

 I have indeed entered the rational domain. This seems permissible, however, 

 because I have proved by previous experiments that the development of any 

 two differentiating characteristics proceeds independently of any other 

 differences. Finally, regarding my statements on the differences among the 

 ovules and pollen cells of the hybrids; they also are based on experiments. 

 These and similar experiments on the germ cells appear to be important, for I 

 believe that their results furnish the explanation for the development of hy- 

 brids as observed in Pisum. These experiments should be repeated and 

 verified. 



I regret very much not being able to send your honor the varieties you 

 desire. As I mentioned above, the experiments were conducted up" to and 

 including 1863; at that time they were terminated in order to obtain space 

 and time for the growing of other experimental plants. Therefore seeds from 

 those experiments are no longer available. Only one experiment on differences 

 in the time of flowering was continued; and seeds are available from the 1864 

 harvest of this experiment. These are the last I collected, since I had to aban- 

 don the experiment in the following year because of devastation by the pea 

 beetle, Bruchus pisi. In the early years of experimentation this insect was only 

 rarely found on the plants, in 1864 it caused considerable damage, and ap- 

 peared in such numbers in the following summer that hardly a 4th or 5th 



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