D.—ZOOLOGY. 77 
with a new meaning—are subject to Mendelian inheritance, and to the 
related belief that small variations are not inherited at all. But towards 
the end of this period the foundations of the controversy vanished, for as 
Prof. H. 8. Jennings® pointed out in 1917, the work of W. E. Castle and 
T. H. Morgan proved that the smallest characters are hereditary, so that 
‘the objections raised by the mutationists to gradual change through 
selection are breaking down as a result of the thoroughness of the mutation- 
ists’ own studies.’ To give a single illustration—between a red-eyed and 
a white-eyed fruit-fly (Drosophila) seven gradations of colour intervene, 
each of them ‘ heritable in the normal Mendelian manner.’ Furthermore, 
in the middle member of this series, ‘ Bridges has found seven modifying 
factors, each of which alters its intensity and gives rise to a secondary 
grade of colour. Now each [all] of these modifying factors are described 
“ specifically as mutations ; as actual changes in the hereditary material.” ’ 
The author finally concludes that ‘ Evolution, according to the typical 
Darwinian scheme, through the occurrence of many small variations and 
their guidance by natural selection, is perfectly consistent with what 
experimental and paleontological studies show us’; indeed, it appears 
to him to be ‘ more consistent with the data than does any other theory,’ 
a conclusion confirmed by Dr. R. A. Fisher’s recent work, ‘ The Genetical 
Theory of Natural Selection.’ Mendelian heredity also provided an 
effective answer to a difficulty by which Darwin had been greatly troubled 
—the supposed ‘swamping effect of intercrossing’ on which Fleeming 
Jenkin had written a powerful article. Moreover, it cannot be doubted 
that Mendelian research, by demonstrating the paramount importance of 
germinal qualities, played a great part in promoting the general acceptance 
of Weismann’s teaching. 
A mistaken belief prevailed in the early years of the Mendelian re- 
discovery that a new theory of evolution had been revealed to the world 
and that Darwinism had been abandoned. The true position was 
emphatically stated by Miss E. R. Saunders at the Cardift Meeting in 1920 
—‘ Mendelism is a theory of heredity ; it is not a theory of evolution.’ 
I need not dwell upon the palzontological evidence for continuous 
evolution, as Prof. Osborn is here and we shall soon have the pleasure of 
listening to one who can tell us of the conclusions to be inferred from the 
matchless record of past ages in the great museum of which he is the 
Director. 
The important subject of Geographical Races or Sub-species will be 
discussed next Tuesday, and I will now only refer to the splendid work of 
the Tring Zoological Museum under the guidance of Lord Rothschild, Dr. 
Hartert and Dr. Jordan, and the conclusions published in their journal 
in 1903.1° ‘Geographical varieties . . . represent various steps in the 
evolution of daughter-species’; and ‘ whoever studies the distinctions of 
8 Journ. Washington Acad. Sci., vol. vii., No. 10, May 19, 1917, p. 281; American 
Naturalist, vol. li., May 1917, p. 301. The statements here reproduced are quoted 
from a brief summary of these two papers in Proc. Ent. Soc. Lond., 1917, p. Ixxxv. 
9 North British Review, June 1867. 
10 Nov. Zool., vol. x., 1903, p. 492. 
