10 



THE AMERICAN NATURALIST 



[Vol. LV 



dence were needed, it may be found in the fine series of 

 experiments lately published by Heribert-Nilsson" in 

 Salix, which, contrary to the belief long ago entertained 

 by Wichura, show that, when Fo families are raised in 

 adequate numbers, very striking segregation occurs in 

 the species-crosses. Many geneticists are inclined to the 

 view that segregable characters should be pictured as 

 implanted on an irreducible base which is outside the 

 scope of segregation, but no means have yet been devised 

 for testing the reality of this conception. 



2. The Moment of Segregation 

 The next question is to determine when in the various 

 life-cycles segregation can occur. Obviously it is a phe- 

 nomenon of cell-division. If we knew nothing of the 

 genetics of plants we might confidently adopt the view 

 which Morgan has so successfully developed, that nor- 

 mal segregation and redistribution happen exclusively 

 in the process of reduction. Though unconvinced, I can 

 not deny that linkage and crossing-over may well be 

 represented provisionally, as effected during synapsis. 

 The scheme previously offered by Punnett and myself 

 as a diagrammatic plan capable of representing these 

 phenomena is certainly far less attractive. There is evi- 

 dence that in certain plants, e.g., Matthiola, the composi- 

 tion of the families derived, from single pods shows very 

 great and perhaps irregular fluctuations, and the normal 

 ratios for those families is only found by taking the 

 average of many, but no sufficient statistical examination 

 of such cases has yet been made. In some suitable case 

 estimations of the offspring derived from individual 

 antliers miuiit be of value in tliis connection. Renner, 

 by ('.\;iininiii,u' the stai'chcs of tln' pollen grains in 

 (Knotlu'i-as, has lately made \ i>iltlc tliat dimorphism, of 

 wiiicli we had. prcx ioii-l \ uviictir i-i-oof, and perhaps this 

 novel and ^Irikin- ul.MM-vat i..n miu'hfbe used for the 

 purpose of niappiim- the di-trihutioii of such a character 



6 Lund's "Uuivt'rsitets Arsskrift, N.F.," Avd. 2, Vol. 14, November 28, 

 1918. 



