559 
Gates.—The Trisomic Mutations of Oenothera. 
somes, at least in the haploid series, all differed from each other through¬ 
out. This assumption is quite possibly too extreme, at least for plants. It 
appears probable that in animals, which are more highly integrated 
organisms than plants, the various chromosomes are differentiated from each 
other throughout their length. But even here there may be a common sub¬ 
stratum for each chromosome which has remained undifferentiated in an 
evolutionary sense. It is reasonable to suppose that the amount of differ¬ 
ence between chromosomes in one organism may be much greater than in 
another. We know this to be the case as regards sizes and shapes of 
chromosomes, since in some organisms the chromosomes are all visibly alike, 
while in others they show many constant differences. It is probable that 
there are equally great differences as regards chemical differentiation, which 
is presumably at the basis of mutations of the Drosophila type. On the 
other hand, it seems necessary to assume some difference between the 
chromosomes of every organism, for if there were no such difference it is 
difficult to see how they would maintain their identity, as they appear to do, 
while passing through the resting stage of the nucleus. But it does not 
necessarily follow that this difference is sufficiently great so that replacement 
of one or two chromosomes by duplicates of others would produce a non- 
viable result, or one which produced a new genus or family. It may be, for 
example, in Oenothera that its generic and family characteristics are equally 
represented in all the chromosomes, while minor differences may have 
arisen more recently through mutations of the type represented by rnbri* 
calyx and brcvistylis. 
An analogy may make this general position clearer. It seems evident 
that the differentiation of the sexes is much greater in some organisms than 
in others. Thus in Hemiptera the relation of sex differentiation to 
dimorphism of the X and Y chromosomes seems clear. On the other 
hand, Schaffer ( 1918 ) has given good reasons for the conclusion that in 
hermaphroditic flowering plants sex differentiation has ‘absolutely nothing 
to do with segregation or association of chromosomes or allosomes In 
a similar way I believe that there is evidence in the Oenotheras that the 
chromosomes are much less differentiated than in animals and some plants, 
i. e. the undifferentiated substratum occupies a greater portion of the whole 
chromosome. And the occurrence of such a large number of trisomic 
mutants in Oenothera , a number of which have interchangeable relationships 
with each other, seems to find its explanation in further assortments of non- 
homologous chromosomes. It seems unlikely that this process could go 
further than the production of forms with four members of one chromosome 
and none of another. 
The almost complete sterility of trisomic forms in Oenothera renders the 
solution of these problems almost impossible, but it is to be hoped that in 
Datura, where there is greater viability and where there are also more 
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