THE 



AMERICAN NATURALIST 



Vol. LV. January-February, 1921 No. 636 



GENETIC SEGREGATION^ 



W. BATESON, M.A., F.R.S. 



Director of the John Inxes Horticultural Institution 

 The later developments of Mendelian analysis have 

 been in the main an attempt to elucidate the scope and 

 nature of segregation. Mendel proved the existence of 

 characters determined by integral or unit factors. Their 

 integrity is maintained by segregation, the capacity, 

 namely, to separate unimpaired after combination with 

 their opposites. Our first aim has been to discover spe- 

 cifically what characters behave in this way, whether 

 there is any limit to the scope of segregation, or any 

 characters or classes of characters which are determined 

 by elements unablo to seiiregate simply. The second ob- 

 ject has l)oon to dccido the time and place in the various 

 life-cycles at wliidi .st'-rc-ation occurs. It is with the 

 latter problem that I i)ropose to deal more particularly 

 in this lecture, but a brief consideration of the range of 

 characters, subject to segregation, is appropriate. 



1. The Scope of Segregation 

 Of the classes of features l)y which animals and plants 

 are distingmshed, most have now been shown to be de- 

 pendent'On segregable elements. It is perhaps notice- 

 able that we have no quite clear proof that the factors 

 governing differences in number, meristic characters in 

 the strict sense, commonly behave so simply as those 

 determining other cliaracteristics. There are examples 

 of parts repeated in srri.->. >ucli :i> tlie .'xtra toe of the 



