JEFFREY: EVOLUTION BY HYBRIDIZATION 301 
ously be most easily estimated and varies in proportion from a small 
percent to complete sterility. The sterility resulting from hybrid- 
ization should not be confused, as sometimes happens, with sterility 
arising from purely physiological causes. For example the common 
horse radish, Lilium bulbiferum and L. candidum, under ordinary 
conditions do not set seed, by reason of the fact that the assimilates 
are too strongly determined to the subterranean parts to permit of 
the necessary materials being set free for the formation of seeds. It 
has been found however that the girdling the top of the subterranean 
stem in the horse radish or cutting off the flowering axis in the case of 
the lilies, brings about the formation of normal seeds. Similarly very 
marked climatic change or subjection to starvation or other extremely 
unfavorable physiological conditions results in the degeneracy of 
reproductive as well as other parts. Conditions like these are, how- 
ever, very easily distinguished from the sterility normally resulting 
from hybridization. 
Sterility in hybrids is of particular interest from the genetical 
standpoint because it more or less completely upsets the expectancy of 
Mendelian ratios in cultures of the offspring of species hybrids. This 
is doubtless one of the causes why the Mendelians have in general 
manifested so little interest in the genetical study of hybrids between 
natural species. Obviously however if the crossing of species in nature 
is a common cause of the origin of new species this part of the evolu- 
tionary field cannot be safely neglected if we are to reach any broad 
and permanently valid conclusions as the modus operandi of the origin 
of species. 
Another feature in the organization of hybrids is the frequent in- 
crease in the typical generic chromosome number as a consequence of 
crossing. For example we find in the much crossed oriental species of 
Chrysanthemum a number of chromosomes in the gametic nuclear 
divisions varying from 9 (the normal) to 18, 27, 36, and even 45, in 
other words, two, three, four and five times the normal gametic 
number. Similarly in another compositaceous genus Dahlia we find 
in the species D. coronata sixteen chromosomata in gametophytic 
divisions while in the hybrids between D. variabilis and D. coccinea 
thirty-two chromosomes have been enumerated. One further example 
will point the situation. In the monotypic Liriodendron and in certain 
species of Magnolia, nineteen gametophytic chromosomes have been 
counted, while in M. soulangeana (suspected of hybrid origin) as well 
as M. Yulan and M. grandiflora twice that number or more of chromo- 
somes have been observed. If we contrast the situation in these 
examples with that presented by the genera Pinus and Lilium, which 
are not at all prone to hybridism, we note a curious contrast. In the 
