TRANSACTIONS OF SECTION D, 583 
view, as it seems to me, is the fact that somatic divisions appear sometimes to 
segregate allelomorphs, as in the case of Datura fruits, and some colour-cases. 
In concluding this brief notice of the complexities of segregation I may call 
attention to the fact that we are here engaged in no idle speculation. For it is 
now possible by experimental means to distinguish almost always with which 
phenomenon we are dealing, and each kind of complication may be separately 
dealt with by a determination of the properties of the extracted forms, Illustra- 
tions of a practical kind will be placed before you at a subsequent meeting. 
The consequence of segregation is that in cases where it occurs we are rid of 
the interminable difficulties which beset all previous attempts to unravel heredity. 
On the older view, the individuals of any group were supposed to belong to an 
indefinite number of classes, according to the various numerical proportions in 
which various types had entered into their pedigree. We now recognise that 
when segregation is allelomorphiec, as it constantly is, the individuals are of three 
classes only in respect of each allelomorphic pair—two homozygous and one 
heterozygous. In all such cases, therefore, fixity of type, instead of increasing 
gradually generation by generation, comes suddenly, and is a phenomenon ot 
individuals, Only by the separate analysis of individuals can this fact be proved. 
The supposition that progress towards fixity of type was gradual arose from the 
study of masses of individuals, and the gradual purification witnessed was due in 
the main to the gradual elimination of impure individuals, whose individual 
properties were wrongly regarded as distributed throughout the mass. 
We have at last the means of demonstrating the presence of integral 
characters. In affirming the integrity of segregable characters we do not declare 
that the size of the integer is fixed eternally, as we suppose the size of a chemical 
unit to be. The integrity of our characters depends on the fact that they can be 
habitually treated as units by gametogenesis. But even where such unity is 
manifested in its most definite form, we may, by sufficient searching, generally find 
a case where the integrity of the character has evidently been impaired in gameto- 
genesis, and where one such individual is found the disintegration can generally 
be propagated. That the size of the unit may be changed by unknown causes, 
though a fact of the highest significance in the attempt to determine the physical 
nature of heredity, does not in the least diminish the value of the recognition ot 
such units, or lessen their part in governing the course of Evolution. 
The existence of unit-characters had, indeed, long been scarcely doubtful to 
those practically familiar with the facts of variation,' but it is to the genius of 
Mendel that we owe the proof, We knew that characters could behave as 
units, but we did not know that this unity was a phenomenon of gametogenesis. 
He has revealed to us the underworld of gametes, Henceforth, whenever we see 
a preparation of germ-cells we shall remember that, though all may look alike, 
they may in reality be of many and definite kinds, differentiated from each other 
according to regular systems. 
Numerical Relations of Gametes and their Significance. 
In addition to the fact of segregation, Mendel’s experiments proved another 
fact nearly as significant ; namely, that when characters are allelomorphic, the 
gametes bearing each member of a pair generally are formed in equal numbers by 
the heterozygote, if an average of cases be taken. This fact can only be regarded 
as a consequence of some numerical symmetry in the cell-divisions of gameto- 
genesis. We already know cases where individual families show such departure 
from normal expectation that either the numbers produced must have been un- 
equal, or subsequent disturbance must have occurred. But so far no case is known 
Jor certain where the average of families does not point to equality. 
The fact that equality is so usual has a direct bearing on conceptions of the 
physical nature of heredity. I have compared our segregation with chemical 
1 Cp. De Vries, Intracellulare Pangenesis, 1889, 
