


. 
LOTSY, CURRENT THEORIES OF EVOLUTION. 399 
We now know that not MENDEL, but NäGELI was deceived, that 
his Hzeracia-hybrids owed their apparent constancy to apogamy, and 
that the law of segregation discovered by MENDEL-was not limited to 
hybrids between so called varieties only, but extended to at least 
some species-hybrids. 
Not however to all; evidently mendelian segregation — depending, 
as it does, on a regular interchange of the members of a pair of 
chromosomes — can apply fully only to hybrids between forms whose 
number of chromosomes is equal. 
Other kinds of hybrids must exist when forms, differing in 
chromosomenumbers are crossed and such have actually come to 
light. TäCKHOLM, and simultaneously and independantly from him, 
Miss BLACKBURN and Dr. Harrison have shown that several Rosae 
are very peculiar hybrids of this kind, so that TacKHOLM could 
even show that nearly all the Rosae, growing spontaneously in 
Europe, North Africa and Western Asia, which up to the time of 
his investigation, were all considered as good species, are, in fact, 
very ancient hybrids of a very peculiar kind. I will illustrate this 
by one example. The ordinary Rosa canina of Europe possesses 
35 chromosomes, 14 of these are arranged in pairs, 21 are single. 
Rosa canina therefore possesses 7 hivalent chromosomes and 21 
univalent ones, or, easier expressed, in BLAKESLEE’S terminology, 
7 disomes and 21 singles: 
The divisions in the pollen mothercells occur in such a way that 
the four ordinary microspores contain 7 chromosomes, derived 
from the disomes and but a few or none of the singles. All the 
other microspores arisen from the pollen mothercell get derivatives 
from the singles only and degenerate. This is the cause of the large 
amount of abortive pollen in the wild Rosae. In the makrospores 
the reduction usually takes place in such a way that the eggcells 
obtain 28 chromosomes, 7 derived from the disomes and all of the 
21 singles. By selffertilisation the original number of 35,7 disomes 
and 21 singles, will therefore be restored in all cases in which 
pollengrains with only 7 chromosomes, derived from the disomes, 
are involved. In thoses cases in which pollengrains with a larger 
number of chromosomes are involved, the number of chromosomes 
‘would of course be larger than 35. There is however no necessity 
to assume that the latter actually ever are involved, as it is not at all 
