1893. HYBRIDITY IN PLANTS. 209 
wheat and rye belonging to different genera. Between different kinds 
of barley he obtained two artificial and six natural hybrids; in no 
case did he succeed in fertilising a two-rowed barley by pollen of a 
variety with a larger number of rows. Among oats, five natural but 
no artificial hybrids were obtained. Very few natural hybrids occur 
among peas ; with different varieties of beet crossing is much more 
common. Rimpau states that if a new form exhibits great variability 
among its descendants, it is probably hybrid; while, on the other hand, 
it is most likely a spontaneous variety if the descendants maintain 
great constancy. 
The Darwinian hypothesis that a sexual mode of reproduction is 
absolutely necessary to the maintenance of the higher forms of life, 
and that continual propagation by non-sexual methods must result in 
deterioration and ultimate extinction, has not been allowed to pass 
unchallenged during recent years. Mdébius in Germany and Meehan 
in America have pointed out the great length of time, extending in 
some cases to thousands of years, during which some plants have 
been continuously propagated by non-sexual methods without apparent 
deterioration or increased liability to disease. This is the case 
with many fruit-trees, such as the fig, date-palm, banana, yam, 
batatas, and olive, and with the Canadian water-weed, Elodea 
canadensis, imported into this country from America, the male plant of 
which is still unknown here; though it is asserted by others that this 
pest is now gradually dying out from our rivers and canals. On the 
other hand, the weeping-willow and the Lombardy poplar—varieties 
which never produce seed, and which are propagated solely by 
cuttings—do appear to be threatened with extinction, owing to their 
abnormal liability to disease. 
We now come to the second part of our subject, and, as we have 
already said, the more difficult one—the nature of the process of 
fertilisation itself. In general terms, fertilisation, or fecundation, may 
be defined as the union of the elements of an active male with those of 
a passive female cell, the result being the production of an embryo 
which develops into a new individual. In the animal kingdom, the 
only mode of reproduction in all the higher forms is a sexual one, 
non-sexual propagation having survived only in the lower types. In 
the vegetable kingdom, the two modes work side by side in the 
higher forms, while in the lowest forms sexuality is unknown. It 
becomes, therefore, a problem of great interest to determine in what 
way the sexual mode of reproduction arose among plants. Much 
light is thrown on this question by the phenomena which have been 
observed among Algz. A very common mode of propagation among 
our ordinary fresh-water Alge is by means of non-sexual zoospores or 
Sswarmspores, minute flagellate bodies which escape from the cells of 
the parent plant, move about in the water with great rapidity, and 
then finally come to rest, withdraw their cilia, and develop into new 
plants. Among the brown sea-weeds, there is a great variety in the 
P 
