230 SCIENCE AND PHILOSOPHY OF THE ORGANISM 



on the fact that there are many exceptions to his rule. 

 In quite a number of cases the hybrids are of one or more 

 types, which remain constant : there is no splitting at all 

 in the second generation. But that does not affect the rule 

 of Mendel in those cases where it is true. Where there 

 is a " splitting " in the second generation, there also are the 

 numerical proportions stated by Mendel ; there never are 

 other relations among the numbers of individuals of the 

 mixed and of the two pure types than those given by his 

 rule. I regard it as very important that this real meaning 

 of Mendel's principle should be most clearly understood. 



From the fact of the splitting of hybrids in the second 

 generation most important consequences may be drawn for 

 the theory of inheritance ; the split individuals, if crossed 

 with each other, always give an offspring which remains 

 pure ; there is no further splitting and no other change 

 whatever. The germ-cells produced by the split individuals 

 of the second generation may therefore be said to be 

 "pure," as pure as were those of the grandparents. But 

 that is as much as to say that the pureness of the germ- 

 cells has been preserved in spite of their passing through 

 the " impure " generation of the hybrids, and from this fact 

 it follows again that the union of characters in the hybrids 

 must have been such as to permit pure separation : in 

 fact, the germ-cells produced by Mendelian hybrids may 

 hypothetically be regarded as being pure themselves. 1 



We have not yet considered one feature of all experi- 



1 This hypothesis was first suggested by Sutton and is at present held by 

 orthodox Mendelians ; but probably things are a little more complicated in 

 reality, as seems to be shown by some facts in the behaviour of so-called 

 "extracted recessives." In Morgan's Experimental Zoology, New York, 1907, 

 a full account of the whole matter is given. 



