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TRANSACTIONS OF SECTION D. 581 
missed. As Mendel immediately proved in the case of peas, and as we have now 
seen in many other plants and animals, it is often impossible to distinguish by 
inspection individuals whose genetic properties are totally distinct. Breeding 
gives the only test. 
Segregation, 
Where the proper precautions have been taken, the following phenomena have 
been proved to occur in a great range of cases, affecting many characters in some 
thirty plants and animals, The qualities or characters whose transmission in 
heredity is examined are found to be distributed among the germ-cells, or 
gametes, as they are called, according to a definite system. This system is such 
that these characters are treated by the cell-divisions (from which the gametes 
result) as existing in pairs, each member of a pair being alternative or 
allelomorphic to the other in the composition of the germ. Now, as every 
zygote—that is, any ordinary animal or plant—is formed by the union of two 
gametes, it may either be made by the union of two gametes bearing similar 
members of any pair, say two blacks or two whites, in which case we call it 
homozygous in respect of that pair, or the gametes from which it originates may 
be bearers of the dissimilar characters, say a black and a white, when we call the 
resulting zygote heterozygous in respect of that pair. If the zygote is homozygous, 
no matter what its parents or their pedigree may have been, it breeds true 
indefinitely unless some fresh variation occurs. 
If, however, the zygote be heterozygous, or gametically cross-bred, its gametes 
in their formation separate the allelomorphs again, so that each gamete contains 
only one allelomorphic character of each pair. At least one cell-division in the 
process of gametogenesis is therefore a differentiating or segregating division, out 
of which each gamete comes sensibly pure in respect of the allelomorph it carries, 
exactly as if it had not been formed by a heterozygous body at all. That, 
translated into modern language, is the essential discovery that Mendel made. 
It has now been repeated and verified for numerous characters of numerous 
species, and, in face of heroic efforts to shake the evidence or to explain it 
away, the discovery of gametic segregation is, and will remain, one of the lasting 
triumphs of the human mind. 
In extending our acquaintance of these phenomena of segregation we encounter 
several principal types of complication. 
Segregation Absent or Incomplete.—From our general knowledge of breeding 
we feel fairly well satisfied that true absence of segregation is the rule in certain 
cases. It is difficult, for instance, to imagine any other account of the facts 
respecting the American Mulattos, though even here sporadic occurrence of 
segregation seems to be authenticated. Very few instances of genuine absence of 
segregation have been critically studied. The only one I can cite from my own 
experience is that of Pararge egeria and egeriades, ‘ climatic’ vaces of a butterfly. 
When crossed together, they give the common intermediate type of North-Western 
France, which, though artificially formed, breeds in great measure true. This 
crossed back with either type has given, as a rule, simple blends between 
intermediate and type. My evidence is not, however, complete enough to 
warrant a positive statement as to the total absence of segregation, for in the few 
families raised from pairs of artificial intermediates some dubious indications of 
segregation have been seen. 
The rarity of true failure of segregation when pure strains are crossed may be 
judged by the fact that since the revival of interest in such work hardly any 
thoroughly satisfactory cases have been witnessed. The largest body of evidence 
on this subject is that provided by De Vries. These cases, however, present so 
many complexities that it is impossible to deal with them now. While so little 
is definitely known regarding non-segregating characters, it appears to me 
premature to attempt any generalisation as to what does or does not segregate. 
Most of the cases of failure of segregation formerly alleged are evidently 
epurious, depending on the appearance of homozygotes in the second genera- 
tion (F,). : 
