Significance of the Available Evidence. 601 
the appearance of transitional forms and only reveal 
their true nature when tested by breeding experiments. 
Several of the critics who have expressed themselves 
more or less favorably on my theory, have pointed out 
that the greatest danger for it lies in this very point. In 
a very clear and concise summary of the doctrine of muta- 
tion MAC DOUGAL has expressed himself as follows : "The 
greatest misunderstanding which may likely arise in the 
consideration of these results will be that founded on the 
error of confusing fluctuating variability and mutabil- 
ity/' 1 
The distinction between species-forming and fluctu- 
ating variability was first derived by DARWIN from his 
theory of pangenesis, and this may perhaps explain the 
antipathy which so many investigators bear towards it. 2 
The great majority of writers assume that fluctuating 
as well as discontinuous variability play a part in the 
formation of species. 3 This view of DARWIN, which 
under WALLACE'S influence gradually shifted into the 
background, has in latter years come again prominently 
to the front; and the various investigators concede here 
a less or there a greater share to discontinuous variations 
or mutations, according to their preconceptions and their 
experience in investigation. This long series of shades 
of opinion would seem to indicate that we are not con- 
cerned here with an independent principle, but with a 
gradual change of opinion from the prevailing theory 
1 D. T. MAcDoucAL, The Origin of Species by Mutation, Torreya, 
1902, Vol. II, p. 99. 
2 See Intraccllular Pangenesis, e.g., p. 214 (English ed.), and 
Ber. d. d. Ges., 1900, XVIII, p. 83. 
3 VON WETTSTEIN has published a useful summary of his views 
in the form of a lecture delivered to the Scientific and Medical Asso- 
ciation at Karlsbad, and entitled Dcr Neo-Lamarckisuius und seine 
Beziehungen sum Danvinismus, 1903. 
