HEREDITY AND EVOLUTION 209 
Yet certainly neither cross-fertilization nor even a sexual 
process is necessary for successful reproduction. Among the 
lower animals there are bisexual species and also species in 
which the eggs develop without fertilization. Both of these types 
of animals seem vigorous and successful. There are numerous 
seed plants in which a sexual fusion is unknown and which pro- 
duce fertile seeds. This may result either from the development — 
of eggs without fertilization or from the growth of a vegetative - 
cell into the embryo sac and its subsequent development into an 
embryo. As has been previously pointed out, plants which are 
reproduced vegetatively or which are habitually self-fertilized 
seem as vigorous as those that are reproduced sexually. 
While the sexual process is thus not necessary for successful 
reproduction, this method does afford great advantage in the 
production of variations upon which natural selection can act. 
If a favorable mutation appeared in an individual of a species 
and another favorable mutation in another individual, these 
two mutations could be combined only as a result of the sex- 
ual process. One advantage of the sexual process would seem 
to be the production of new types of individuals which may 
be better fitted to survive. 
EVOLUTION 
The production of anything by the process of gradual unfold- 
ing or gradual change is known as evolution. In this sense we 
speak of the evolution of a machine, the evolution of civilization, 
the evolution of a nation, the evolution of the world, or the 
evolution of the universe. 
The present configuration of the earth is known to be due 
to the accumulation of the same kind of gradual changes as 
are going on around us at the present time. Streams gradually 
deepen their beds, while soil is washed from hillsides into rivers 
and then carried into lakes or seas. In this way the earth has 
become cut into ridges and valleys. In some places the land is 
rising, while in others it is sinking. Earthquakes and volcanic 
