408 LOTSY, CURRENT THEORIES OF EVOLUTION. 
contact with another Numida-species and hybridizes with it;its so 
called varieties therefore are, just as the varieties of so many New 
England Violae, as shown by BRAINERD, alterations caused by 
crossing. 
Taking all in all, I have no doubt, that Hess has been 
and still is an important factor of evolution. 
Some objections may occur to you to accept it as the only 
cause and I myself do not think that it explains all diversity. 
Allow me to consider first the objections which have come to 
my knowledge and then to explain at which point, to my way of 
thinking, the influence of hybridisation stops. 
The first objection which naturally presents itself, is that devi- 
ations have been observed in pure cultures from organisms, such 
as fungi and bacteria, which multiply asexually only. 
A case which appeared especially convincing was a form of 
Botrytis cinerea with colorless sclerotia obtained from a single 
spore strain of Botrytis cinerea with normal black sclerotia by BRIERLY 
and which was, at first, described by him as.a mutation. Aferwards 
however BRIERLY considered the question whether a single strain 
culture in the case of multinuclear spores, such as Botrytis pos- 
sesses, is a guarantee of purity and concluded that it was not. 
Referring to BURGEFFS crosses of various forms of Phycomyces 
nitens, BRIERLY says: 
„In the asexual form of reproduction spores are delimited within 
sporangial heads into which there have passed an indefinite number 
of nuclei. The multinucleate spores germinate and reproduce the 
coenocytic mycelium. If, therefore, the original hyphae are gene- 
tically impure, this condition will be maintained in all succeeding 
generations, for the sporangiospores merely reproduce the genetic 
condition of the hyphae which give rise to sporangia. Opportuni- 
ty for genetic contamination occurs at sexual reproduction, for this 
process is merely a fusion of two multinucleate gametes to form 
a multinucleate zygospore, which on germination gives rise to a 
coenocytic mycelium containing nuclei of both parental strains. 
There will be an equal chance for both types of nuclei to pass 
into the sporangia and be included in the multinucleate spores. 
A single-spore strain may thus be heterocaryotic. If now this form 
at sexual maturity fuses with a third form, and so on, the genotype 






