406 LOTSY, CURRENT THEORIES OF EVOLUTION. 
taneously, as admirable material for his theory of the origin of 
species by natural selection. 
The proof of the pudding however is in the eating and I cannot 
help feeling, that the pudding served to us, since I first ventured 
to publish my theory, tastes strongly after hybridization. 
While DE Vries in his mutation-theory predicted that the 
cause of the polymorphy in Oenothera, in Draba verna, in Viola 
and in Helianthemum, as well as the nebulae of the older systematists 
in Rubus, Hieracium, Rosa and Salix would prove to be due to 
mutation, we now know that in all these cases, as far as examined — 
and this has been the case with all of them expect Zelianthemum — 
even those who are yet inclined to ascribe part of that polymorphy 
to mutation, must admit that hybridization is responsible for 
by far the greater part of it, while I myself do not think that 
there is any proof for any influence of mutation in any of these 
cases. - 
There is another group of plants which is the despair of taxonomists: 
the Cacti and I was much {gratified to see, during my stay at 
Tucson in Arizona, that in their polymorphy hybridization also, 
to say the least, has played a considerable role. 
Not only could Mac DouGAt show that hybrids between Cacti 
segregate, but he could also show that many at least of the wild 
Cacti are heterozygous to such a degree that he could not use, 
certain Echinocerei for instance, for his experiments to change the 
plasmatic constitution by injection. Moreover there are a number 
of presumably wild hybrids known among the Cacti, as was assured 
to me both by Prof. THORNBER, the authority on the vegetation of 
the Arizona desert, and by Mac DouGaL and the latter’s proof 
of the heterozygosity of wild Cactz whose hybrid nature was not 
suspected, leaves no doubt that this assumption is correct. 
What is more important still, is the fact that Mac DouGAL could 
show, by observations running over a number of years, that the 
highly remarkable diversity in flower color in the case of 
Opuntia versicolor, running from pale yellow, almost white, to 
dark yellow, through pink, bordeaux-red of all shades to deep 
bluish red, is not variable at all, but typical for each individual. 
Now this diversity in color is exactly what we would expect to 
to arise by segregation after a cross and as these Cacti repro- 


