W. Neilson Jones 
i 72 
eyed” flowers are susceptible to the male gametes from “ pin- 
yed ” flowers and immune to male gametes from “ thrum-eyed ” 
flowers. Prom the point of view now under consideration such 
behaviour may be interpreted as a special case of differential 
immunity, other instances of which are known ( e.g ., in the orchids, 
the root system is susceptible to infection by the mycorhizal 
fungus, while the immunity of the shoot system excludes the fungus 
from this part of the plant). 
The difficulty in obtaining seed as the result of pollinating 
one plant species with the pollen from another species is capable 
of a somewhat similar explanation, viz.: the egg-cells are susceptible 
to attack by male gametes of the same species but, in general, 
immune to gametes from another species. It must not be forgotten, 
however, that failure to set seed when species are crossed may be 
due to lack of attraction as well as to the presence of a definite 
immunity on the part of the egg-cell, just as may the freedom of a 
plant against infection by a specific parasite. The variability as to 
sterility found in different varieties of a species finds an obvious 
parallel in the varying susceptibility and immunity of varieties to 
attack by parasites, as is to be seen for example in the “ biologic 
races ” of grasses described by Salmon ( loc . cit.). 
The “ plus ” and “ minus ” races of Phycomyces and other 
members of the Mucorineae investigated by Blakeslee can be 
evidently described in the same terms and may he analogous in 
nature. 
On the present hypothesis the origin and evolution of sexuality 
becomes closely bound up with those of parasitism. The earlier 
forms of life were inevitably holotrophic, and from these were 
evolved organisms whose protoplasm reacted mutually in the 
manner characteristic of parasite and host. An advanced stage 
of such evolution is represented by plants, such as the orchids, in 
which the stimulus of infection by a fungus is necessary before 
development of the seedling can take place, or by those obligate 
parasites which require a specific host. 
The most primitive form of reproduction in unicellular plants 
is by simple fission, and this becomes elaborated into the pro¬ 
duction of asexual spores. Among the green and brown Algae can 
be traced an almost continuous series leading from such spores to 
gametes morphologically, and in many ways physiologically, alike; 
and ultimately to cases where the gametes show differentiation 
into a larger non-motile female cell and a smaller actively free- 
