GENETICS: R. J. STOCKING 
609 
viduals of an abnormal line may be normal, others abnormal in various 
degrees. Different lines show diverse proportions of abnormal indi- 
viduals. It thus becomes possible to test by selection the inheritance 
of degree and kind of abnormahty within a single stock all derived by 
fission from a single parent. 
In long continued uniparental reproduction of any sort, a remarkable 
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 accompanying 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 recom- 
binations of characteristics in the progeny, in Mendelian inheritance. 
In uniparental reproduction, particularly of the vegetative kind, such 
nuclear reductions and recombinations are not known; and the uni- 
formity of the progeny is in agreement with this. These relations, with 
others not necessary to recount here, have given origin to the concep- 
tion of the genotype, as the hereditary constitution, in contradistinction 
to the bodily appearance. The genotype is commonly held not to change 
in vegetative reproduction, or but rarely, and then by marked sudden 
steps, or mutations. In biparental reproduction the genotype does 
indeed change, but seemingly by mere shiftings and recombinations, 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 naturally results 
from the view of the facts 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. On the other hand, 
certain investigators in genetics oppose strongly this rigid view, holding 
that, over and beyond Mendelian recombinations, hereditary variations 
of sHght degree are frequently occurring, so that evolution may well be 
continuous and guided by selection. The recent papers of Castle give 
typical expression to this point of view. 
If hereditary variations are frequently occurring, aside from Mendel- 
ian recombinations, it should be possible to find them in vegetative 
reproduction. Here we are freed from the mixing of types which makes 
these relations so difficult to interpret in biparental reproduction. The 
abnormalities in Paramecium were, therefore, studied mainly with rela- 
