54 REPRODUCTION AND LIFE HISTORY. 
which may befall the “body” which bears them. And, 
finally, in the mixture of two units of living matter which 
have had different histories, an opportunity for new permuta- 
tions and combinations, in other words, for variation, 18 
supplied. Thus it is not surprising to find that the asexual 
method of liberating buds has been replaced in most 
animals by the more economical and advantageous process 
of sexual reproduction. 
SumMMARY OF MopEs OF REPRODUCTION 
A. In Single-celled Animals (Protozoa) 
(1) The almost mechanical rupture of an amoeboid cell, which has 
become too large for physiological equilibrium. 
(2) The discharge of numerous superficial buds at once (e.g. Arcella 
and Pelomyxa). 5 * 
(3) The formation of one bud at a time (very common). 
(4) The ordinary division into two daughter cells at the limit of 
growth. 
(5) Repeated divisions within limited time and within limited space’ 
(a cyst). This results in what is called spore-formation (e.9. 
in Sporozoa). 
B. Ln Many-celled Animals (Metazoa) , 
(Asexual) 
(a) The separation of a clump of body cells, e.g. from the surface of 
some Sponges. (A crude form of budding.) 
(4) The formation of definite buds which may or may not be set free. 
(c) Various forms of fission and fragmentation. 
(Sexual) 
The liberation of special reproductive or germ cells, which have 
not taken part in the formation of the body, and which retain 
the essential qualities of the original germ cell from which the ' 
parent arose. These special germ cells—the ova and sperma- 
tozoa—are normally united in fertilisation, but some animals 
have (parthenogenetic) ova which develop without being 
fertilised. 
Evolution of sex.—A further problem is to account for 
the two facts—(a) that most animals are either males or 
temales, the former liberating actively motile male elements 
or spermatozoa, the latter ‘forming and usually liberating 
more passive egg cells or ova; and (4) that these two 
