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importance of applying the direct methods of experiment to 
these questions was at once widely recognized. Already 
after an interval of only four years, real and solid advances 
in knowledge have been made, and on every hand the range 
of facts explicable on the Mendelian theory has been ex¬ 
tended. Although the work has not yet passed beyond the 
foundations of a great branch of evolutionary science, a 
brief general account of the progress which has been so far 
made may not be without interest, in view of the great 
importance of the subject. 
Mendel’s own work is now fully familiar to the majority 
of naturalists, and for the publication of yet another 
summary of his conclusions an apology would be wanting 
did it not appear that in some quarters the full importance 
of Mendel’s discovery has still failed to be grasped. Thus 
Vernon writing in 1903 (64) still lays stress on the perfection 
or imperfection of dominance as if this were a prime issue, 
whereas the principal law of Mendel, as it is very clearly 
stated by Correns in the paper which Vernon quotes, takes no 
account of dominance at all. 
Pearson again, in the course of a valuable investigation 
into the relation between the principle of gametic purity 
and the law of ancestral heredity (54), entirely ignores 
Mendel’s demonstration of the truth of his hypothesis by 
the process of recrossing with the parental forms. Pearson, 
too, would confine the term “ Mendelian theory ” to the 
particular cases where dominance appears. But it cannot be 
too forcibly stated that the facts of dominance were merely 
incidental to Mendel’s experiments, and constitute no funda¬ 
mental portion of his theory. 
The idea of discontinuity in evolutionary processes is one 
which is now fully familiar through the labours especially 
of Bateson (1) and de Vries (69). All the most recent work 
upon the subject of species has confirmed more and more 
fully the importance of this conception, and the progress 
here dealt with is concerned throughout with the 
