246 Robertson : Theortes of. Evolution. 
are some very curious pairs of qualities which work out accord- 
ing to Mendelian expectations. For instance, when a race of 
wheat known to be highly susceptible to rust disease is crossed 
with one which is immune to the disease, in the first generation 
all the plants are susceptible, in other words, susceptibzlity is 
dominant. In the next generation three-quarters are susceptible 
and one quarter immune ! 
The phenomenon of gametic purity is of far greater theoretical 
importance than that of dominance. There are many cases in 
which the first generation from the cross, instead of showing 
the complete dominance of one or other character, shows some- 
thing quite different, often an apparent reversion to a more 
primitive type. For instance, the cross between an albino 
mouse and a black and white waltzing mouse has been found to 
resemble a wild mouse. Among plants there is a white sweet- 
pea called ‘Emily Henderson’ which has round pollen grains, 
while most sweet-peas have long pollen grains. ‘Emily 
Henderson’ is not, however, a pure race, since some of the 
plants, which in every external character exactly resemble their 
fellows, have /ong pollen grains. If the long-pollened and 
short-pollened ‘Emily Hendersons’ are crossed together the 
long pollen is dominant over the round, but, as regards flower- 
colour, instead of the whiteness which we should expect, we 
get a purple flower with a chocolate standard, which is supposed 
to be the ancestral form from which all sweet-peas have been 
derived! It would seem that the present races of sweet-peas 
are a series of ‘analytical dissociations ’ of the characters of the 
original ancestor, and that when two of them are crossed we 
sometimes get the characters re-combining and giving an 
appearance recalling the ancestral form. 
It is impossible in this extremely brief outline to do more 
than touch upon the fringe of Mendelian work. I have left out 
of account everything but the simplest possible cases. For 
further information I should like to refer the reader to a little 
book on ‘Mendelism,’ by R. C. Punnett (Macmillan), in which 
the present aspects of the subject are clearly discussed. 
VI.—THE MUTATION, THEORY. 
According to the Darwinian theory the Origin of Species 
has been an extremely slow and gradual affair, depending chiefly 
upon the preservation and accumulation by natural selection of 
minute individual variations of a favourable kind, while similar 
Naturalist, 
