404 LOTSY, CURRENT THEORIES OF EVOLUTION. 
This new species E, although a homozygote, has nevertheless 
derived its chromosomes from three different sources and cor- 
sequently is a hybrid, so that even the pure line is not above the 
suspicion of being of a hybrid nature. 
If this hypothesis of WINGE should prove to be correct — and 
the Saccharum case points that way — it would of course em- 
phasize the great role which hybridization has played in evolution. 
No doubt WINGE’s hypothesis very nicely explains the series 
of chromosome numbers within such genera in which each species 
differs from the preceding one by the so called cardinal number 
as f.i. in the case of Chrysanthemum : 
Chrysanthemum coronarium 9 
La leucanthemum 18 
5 morifolium 27 
<5 Decaisneanum 36 
Another fact which speaks in favor of WINGE’s hypothesis is 
the occurrence of pairs of chromosomes of different shape in a 
particular set as f. i. in the case of BLAKESLEE’S Datura stramonium. 
haploid diploid 
extra Large hes 2 L 
large 4 1 8 1 
large Medium 3 M 6 M 
small medium 2 m 4 m 
small 1S 258 
extra small ls 248 
12 24 
Or in that of the three pairs of Drosophila, which differ so 
conspicuously from one another that one might very well imagine 
them to have been derived from three different sources. 
We consequently see that even a pure line is not above the 
suspicion of hybridity, that it may have derived its chromosomes from 
different sources, so that we are only beginning to realise how much 
hybridization, besides mendelian one, may be going on in nature. 
What then is the 
Cause of Evolution. 
Although diversity was the actual starting-point of DARWIN’s Selec- 
tiontheory, all of us were so thoroughly convinced of the necessity 


