TRANSACTIONS OF SECTION D, 587 
Even nearer we come in a certain sweet-pea example, where abortion of 
anthers behaves as an ordinary Mendelian recessive character. By a slight 
exaggeration we might even speak of a hermaphrodite with barren anthers as a 
‘ female.’ 
Consider also how like the two kinds of differentiation are. The occasional 
mosaicism in Lepidoptera, called ‘ gynandromorphism,’ may be exactly paralleled 
by specimens where the two halves are two colour-varieties, instead of the two 
sexes, Patches of Silene inflata in this neighbourhood commonly consist of hairy 
and glabrous individuals,? a phenomenon proved in Lychnis to be dependent on 
Mendelian segregation. The same patch consists also of female plants and herma- 
phrodite plants, Is it not likely that both phenomena are similar in nature? 
How otherwise would the differentiation be maintained? The sweet-pea case I 
have spoken of is scarcely distinguishable from this, I therefore look forward 
with confidence to the elucidation of the real nature of sex—that redoubtable 
mystery. 
We now move among the facts with an altogether different bearing. ‘ Animals 
and Plants under Domestication,’ from being largely a narration of inscrutable 
prodigies, begins to take shape as a body of coherent evidence. Of the old 
difficulties many disappear finally. Others are inverted. Darwin says he would 
have expected ‘from the law of reversion’ that nectarines being the newer form 
would more often produce peaches than peaches nectarines, which is the commoner 
occurrence. Now, on the contrary, the unique instance of the Carclew nectarine 
tree bearing peaches is more astonishing than all the other evidence together ! 
Though the progress which Mendelian facts make possible is so great, it must 
never be forgotten that as regards new characters involving the addition of some 
new factor to the pre-existing stock we are almost where we were. When they 
have been added by mutation, we can now study their transmission ; but we know 
not whence or why they come. Nor have we any definite light on the problem 
of adaptation; though here there is at least no increase of difficulties. 
Besides these outstanding problems, there remain many special points of 
difficulty which on this occasion I cannot treat—curiosities of segregation, obscure 
aberrations of fertilisation * (occasionally met with), coupling of characters, and the 
very serious possibility of disturbance through gametic selection. Let us employ 
the space that remains in returning to the problem of variation, already spoken of 
above, and considering how it looks in the light of the new facts as to heredity. 
The problem of heredity is the problem of the manner of distribution of characters 
among germ-cells, So soon as this problem is truly formulated, the nature of 
variation at once appears. For the first time in the history of evolutionary 
thought, Mendel’s discovery enables us to form some picture of the process which 
results in genetic variation. It is simply the segregation of a new kind of gamete, 
bearing one or morecharacters distinct from those of the type. We can answer 
one of the oldest questions in philosophy, In terms of the ancient riddle, we may 
1 Neglecting minor complications, the descent is as follows:—Lady Penzance 
x Emily Henderson (long pollen) g gave purple F,. In one F, family, with rare 
exceptions, colowred plants with dark axils were fertile, those with light axils having 
6 sterile, whites being either fertile or sterile. The ratios indicated are 9 coloured, 
dk. ax., fertile ¢: 3 coloured, lt. ax., sterile ¢: 3 white, fertile ¢: 1 white, 
sterile ¢. The fertile whites, therefore, though light-axilled (as whites almost 
always are) presumably bear the dark-axil character, which generally cannot appear 
except in association with coloured flowers. This can be proved next year. Some 
at least of the plants with sterile ¢ are fertile on the ? side, and when crossed with 
a coloured light-axilled type will presumably give only light-axilled plants. 
? This excellent illustration was shown me by Mr. A. W. Hill and Mr. A. Wallis. 
A third form, glabrous, with hairy edges to the leaves, also occurs. 
’ In view of Ostenfeld’s discovery of parthenogenesis in Hieraciwm, the possi- 
bility that this phenomenon plays a part in some non-segregating cases needs careful 
examination, 
