IMPORTANCE OF HYBRIDISATION IN THE STUDY OF DESCENT. 281 
as in the spontaneous mutations of (/nothera Lamarckiana. When 
characters recognisable as belonging to the fundamental type of Piswm 
arvense, e.g. purple flowers, spots on the leaves, dark brown seed-skin, 
indent shape of seed, appear suddenly on a cross with an aberrant race 
showing defect of some character, e.g. pink flowers, absence of leaf-spots, 
seed-skin marked with a pattern or transparent, round shape of seeds, the 
case is evidently one of hybrid atavism. In other cases, nevertheless, we 
have certainly to do with actual novelties, in particular with defect- 
mutations, such as albinism, produced by hybridisation. It is therefore 
quite possible that, in the history of organic evolution, hybridisation has 
not rarely resulted in the production of new races, and perhaps in the 
appearance of progressive mutations. It may be remarked parenthetically 
that my observations (as well as those of Saunders and Bateson) on 
Matthiola hybrids and on barley hybrids incline me to doubt whether 
the production of absolutely pure gametes in Mendel’s sense is of general 
occurrence. I suggest rather—at all events in certain cases which must 
be regarded as crypto-hybridisations—that there is a double potentiality 
with substantial prevalency of one or other of the characters, and 
particularly in the case of the recessives, which though of hybrid origin 
breed pure if in-bred ; that there is a latency and not a complete absence 
of the dominant character.* 
_ One must, of course, on the one hand, be cautious in regard to a 
proposition of such far-reaching consequences ; on the other hand, it 
would certainly be dangerous to endorse the doctrine of gametic purity as 
one of which no doubt could be entertained.T 
In so far as the new characters appearing as the result of a cross can 
be regarded as atavistic, they have a phylogenetic significance. Crossing 
in such cases is a means by which characters apparently lost in the 
phylogeny can be reactivated, and it thus throws light on the history of 
the form now existing. On the other hand the capability of a single 
species as regards production for varietal forms—its range of form, as 
it has been called (Goebel, Celakowsky, Heinricher, de Vries)—may be 
determined by study of its hybrid-mutations—just as it may by that of its 
spontaneous variations and mutations. Crossing is thus an experimental 
method by which the condition of the separate characters may be raised 
* In these cases also a fresh cross may be enough to reactivate the original 
parental character which is latent, with irregular Mendelian valency (as dominant, 
co-dominant, recessive or co-recessive) or with polymorphism in the first generation. 
In illustration: hoariness or pigmentation in the case of smooth or otherwise 
pigmented descendants from the cross smooth = hoary, or from white = coloured in 
Matthiola, Tschermak, Bateson. Cp. also observations of Cuénot, Haacke, Guaita, 
Castle (1903), Castle and Allen (1903), Bateson (1903), on mouse-crosses ; as also those 
of Darbishire and Hurst. 
T. H. Morgan, Science xxii. 1905 and Biol. Centralbl. xxvi. 1906, p. 289, advocates 
generally the hypothesis of impurity of the gametes of hybrids (i.e. the double 
potentiality with prevalency or dominance of one character over the other, and a 
production of gametes of both kinds on an average in equal numbers). He would 
regard the extracted dominants which breed true as those in which the recessive 
character is latent, and those in which the recessive character is manifested (the 
extracted recessives) as latent-dominants, thus representing all these segregation- 
products as crypto-hybrids in the sense which I first proposed in October 1903. 
+ I have already advocated this point of view. Beitr. z. Bot. Centralbl. Bd. 16, 
H. 1, 1903, October, pp. 20 and 25. See also Zts. f. das Landw. Versw. in Oesterr. 
1904, p. 23; Arch, f. Rassen wu. Ges. Biol, 2. Jabrg., H. 5 and 6, 1905, 
