THE EFFECT OF CROSSING LINNEONS
On talking with different scientific friends about the views which have gradually developped in my mind about crossing being the origin of species, I have frequently heard the objection — which is pretty generally offered against all theories of evolution — that the effect is inconsiderable, compared with the changes wanted to explain the origin of such different groups, say as fishes, reptils and mammals.
I will return to this question in the next chapter, limiting myself in the present one, to show what the effect of an actually executed cross between individuals belonging to different Linneons has been.
It is impossible to treat here of all the numerous forms which result from such a cross; for this I must refer the reader to a book by the author, on the hybrids between different Linneons of Antirrhinum which would have appeared long ago, if the war had not interfered with the making of the plates.
But as all that is necessary here, is to show the great diversity which can be the result of a cross, I can suffice by restating, what I said at the IVe Conférence de Génétique in Paris in içii and by reproducing some photographs as illustration.
In 1910 Professor BAUR of Berlin succeeded in crossing certain species of Antirrhinum, the hybrids of which proved to be fully fertile. The seeds of two of these hybrid-combinations he very kindly gave tome for further investigation.
One of these combinations was Antirrhinum glutinosum crossfertilised with a peloric form of Antirrhinum majus.
As Antirrhinum glutinosum contains several types, all of which are completely selfsterile, the F1 generation obtained was somewhat polymorphous, though not to a considerable extent. On the whole, it was fairly intermediate between the two parent species.
The F2 obtained from the F1 plants was exceedingly polymorphous; one of the self-fertilised F1 plants gave 255 children, not two of which were alike. They differed in a large number of characters, such as colour, form of flowers, habit of growth, leaf-characters, hoariness, self-fertility, resistance to draught and frost etc. Zygomorphous and peloric flowers were always present, and it appears that the segregation was a complicated mendelian one.
The most interesting results perhaps were obtained from the cross A. glutinosum by a red zygomorphous form of A. majus.
In the F2 of this cross, several remarkable forms occurred, in one f. i. the sepals were coloured and petaloid, another showed several spur-like excressences at the lower lip of the flower and some had flowers astonishingly different from those of the parent-species, resembling more a Rhinanthus than an Antirrhinum and of a type entirely unknown hitherto within this latter genus, as the photographs here reproduced show.
The first photograph shows at the top, indicated as P1 and P2, the two parents crossed.
P1 is the flower of Antirrhinum glutinosum, which is white, except that it has pink striae on the upperlip. The flower indicated by P2, is the dark red flower of Antirrhinum majus with which the former was crossed. Of the two other flowers on this photograph, the one which appeared in the fourth selfed generation (F4), was pure ivory and characterized by the possession of spur-like organs at the lower lip, while the one which appeared in F2 was light pink, and showed beautiful petaloid sepals of the same color but in a lighter shade.
The second photograph shows one flower of a plant of the 2d generation, indicated by F2, which was fleshcolored and resembled the flower of a Rhinanthus, the other flowers, all belonging to the 4th generation (F4), were magenta and showed shapes very different indeed also, from the parent-species crossed.
It seems to me, that this little suffises to show that very unexpected results can arise from a cross, forms so different from the Linneons crossed, that no systematist could possibly say to which of these Linneons they belonged, yes so different, that if their origin were not known, they might easily be referred to different genera.