188 COMPARATIVE ZOOLOGY. 
CHAPTER XIX. 
REPRODUCTION. 
Tr is a fundamental truth that every living organism 
has had its origin in some pre-existing organism. The 
doctrine of “spontaneous generation,” or the supposed 
origination of organized structures out of inorganic parti- 
cles, has not yet been sustained by facts. 
All animals, without exception, arise from eggs. But 
while reproduction by eggs is common to all, it is only one 
Fig. 156.—Reproduction of Infusoria (Vorticel- 
le) by fission or self-division. 
among several modes of 
multiplication. For the 
lowest forms of life not 
only generate by eggs, 
but also by self-diviston 
and budding.” 
Self- division, the 
simplest mode possible, 
is a natural breaking-up 
of the body into distinct 
surviving parts. This 
process is sometimes ex- 
traordinarily rapid, the 
increase of one animal- 
cule (Paramecium) be- 
ing computed at 268 
millions in a month. It 
is most common in the 
Infusoria; but is occasionally exhibited even by the aquat- 
ic and intestinal worms. 
Budding consists, in animals as in plants, in the growth 
