oe 
28 | The Significance of Sex, [Jan. 
There is, however, this difference between the dzo/ogic and the 
social individual,—the latter can be formed by association of units 
at first independent, while in the former the cells are always ge- 
netically related. The metazoa arose from the protozoa by a 
modification in the mode of reproduction by self-division, which 
caused the daughter-cells to remain united. Now, let all the 
other phenomena and forces remain as before, these daughters 
will soon divide again, they will not separate but will go on for a 
considerable period until an aggregate of cells results, then by the 
operation of the principles that produce alternation of generations 
in separated forms and polymorphism in colonies, there will follow 
what we term the differentiation of tissues, and lo, a metazoon. In 
|! asimilar way, among the Metazoa a multisegmented form must have 
arisen from one unisegmented by modified budding or strobilation. 
Natural selection will account for the preservation of forms, but 
the cause and origin of new forms lies in the above laws of or- 
ganization. We are now prepared for the next step in this argu- 
ment. As self-division is the only form of reproduction that 
could give rise to the Metazoa, we understand why this is the 
mode which alone operates during ontogeny. It is also the usual 
mode of reproduction among the Protozoa. Once in a while 
_ under hard conditions of nutrition, etc. (perhaps so only as to 
its origin—Weissmann), the protozoan individual, too feeble to 
fight the battle of life alone, fuses (conjugation with a neighbor 
(sometimes more than one ?), and thus reinvigorated goes on in 
its former way again. Possibly conjugation is only one, though 
the most useful, of several methods by which reyuvenescence can 
be effected; at any rate we can see that by “sexual reproduc- 
tion” we do not mean a new mode of reproduction contrasted with 
the asexual mode, but simply a particular mode of a sexual repro- 
duction preceded by a particular method of rejuvenescence (conju- 
gation, fertilization’). Now when the Metazoa were formed by 
the non-separation of cells produced by binary division, those 
cells that required rejuvenescence were set free that they might 
* Van Beneden, Archives de Biologie, iv. p. 616. 
