PEDIGREE INHERITANCE. 
407 
zygots) : blacks (recessive homozygots) ==1:2:1. — The homozygots will 
breed true, while the successive produce of the heterozygots will again 
show the 1:2:1 ratio. 
Mendel’s Principles succeed in some of the cases, and fail in others. 
To support this theory and make it conformable to facts, it was necessary 
to set up several working hypotheses. Bateson’s Presence and Absence 
Hypothesis, which, likewise, often contradicts façts, belongs to the latter also. 
It is characteristic of the Mendelian manner of researches, that, in 
some cases, ancestral properties are considered as dominant (De Vries), 
and new characteristics as recessive, or vice versa, in others ; or the presence 
of a certain property is looked upon as dominant, and the absence of it, as 
recessive (Bateson) ; or the presence of a factor which prevents the appear¬ 
ance of a property (the «Hemmungsfaktor» of Lang) is considered to be 
dominant, and the absence of this curious factor, which does not prevent 
the showing up of the respective property, as a recessive character; or 
finally, the necessary, sometimes extra-ordinary, increasing or diminishing 
of the numbers of the pairs of factors which ought to be supposed, is cha¬ 
racteristic of the aim which is directed towards forcing the facts into the 
Mendelian fixed limits. 
In the sense of Mendel’s Doctrine, the dominant or recessive cha¬ 
racters of the single properties can only afterwards be proved by experi¬ 
ments, the cause of the behaviour of the characters being passed over, and 
being inexplicable by this doctrine. Furthermore, in this sense, the com¬ 
plex (?) properties, as polyhybrid crosses, must be taken asunder into unit (?) 
characters, into monohybrid crosses, and the suppositions of the very (?) 
numbers of the pairs of factors, which are at work, must be based, after¬ 
wards, upon the results of the experiments too. (Thus e. g. the colour- 
inheritance of mice is nowadays generally supposed to be the consequence 
of the cooperation of about. 10 pairs of factors.) It cannot be expected that 
the inheritance of the very complex properties of highly differentiated 
organisms, their total heritage, could be deduced consistently, and according 
to the facts, in all the cases, by means of the Mendelian Theory. 
Let us suppose, that from a pure grey race, under the influence of 
certain natural causes, two new breeds, a white and a black, have sprung 
apart, which, during a certain time, and under unchanged conditions, and 
up to a certain degree, became constant, breeding true. 
a) The cross between pure-bred grey and pure-bred white is shown 
by the following pedigree : 
grey 
Ï 
grey 
X 
grey 
! 
white 
