INHERITANCE IN ABNORMALITIES 413 



able constancy in hereditary characteristics has been generally 

 reported. All the progeny thus coming from a single parent 

 have seemed uniform in their hereditary characteristics, though 

 they may differ in their bodily appearance. And this is quite 

 in agreement with the known cytological processes accompany- 

 ing the two types of reproduction. In biparental reproduction 

 there is a reduction and recombination of the nuclear elements, 

 of precisely the same sort as the variation and recombinations of 

 characters in the progeny in Mendelian inheritance. In uni- 

 parental reproduction, particularly of the vegetative kind,^ such 

 nuclear reductions and recombinations are not known; and the 

 uniformity of the progeny is in agreement with this. 



These relations, with others not necessary to recount here, 

 have given origin to the conception of the genotype as the 

 hereditary constitution, in contradistinction to the bodily appear- 

 ance. The genotype is commonly held not to change in vegeta- 

 tive reproduction, or but rarely, and then by marked sudden 

 steps, or mutations. In biparental reproduction the genotype 

 does indeed change, but seemingly by mere shif tings and recom- 

 binations, in numerically predictable ways; so that the relations 

 here are quite in agreement with the condition sketched above 

 for uniparental reproduction. 



A somewhat rigid, stereotyped scheme of heredity naturall}^ 

 results from the view of the facts as just set forth; in particular, 

 evolution by gradual change, guided by natural selection, appears 

 to be excluded. This becomes still more marked if we conclude 

 with Bateson ('14) that all mutations consist in the dropping 

 out of factors. However, certain investigators in genetics oppose 

 this rigid view, holding that, over and beyond Mendelian recom- 

 binations, hereditary variations of slight degree are frequently 

 occurring, so that evolution maj^ well be continuous and guided 

 by selection. The recent papers of Castle give typical expres- 

 sion to this point of view. 



- In their recent description of endomixis during the vegetative reproduction 

 of P. aurelia, Woodruff and Erchnann observed neither reduction nor fusion of 

 nuclear elements. 



