Fertilisation , Apogamy and Parthenogenesis. 151 
purpose of fertilisation is still very doubtful. Weismann has 
attacked the “rejuvenescence” theory of fertilisation and considers 
that the primary purpose of the process is to bring about variations ; 
the need of fertilisation to renew the waning vigour of the proto¬ 
plasm he considers to be secondary in nature and to have been later 
acquired. Darwin, Spencer, Oscar Hertwig, Strasburger and 
others believe on the other hand that the chief importance of ferti¬ 
lisation is to prevent too great variation and to keep the species true 
to the normal. 
If the view, first put forward in an unsatisfactory form by 
Cannon, and later in an acceptable form by Sutton, 1 -—that it is the 
reduction-division which leads to a segregation of maternal and 
paternal characters in accordance with Mendel’s law,—be correct, 
it is clear that that division and the ensuing fertilisation would he a 
potent cause of variation ; for of the gametes arising from an indi¬ 
vidual whose parents differed in a considerable number of characters 
hardly any two would be alike, and these would fuse with gametes 
equally dissimilar. 
There can be little doubt, however, that, as other workers have 
stated, exogamous fertilisation tends not only to produce small 
variations, but also to keep them within narrow limits by a constant 
process of mixing; thus both views may be accepted. But it 
must be pointed out that the generally accepted view that individuals 
formed asexually are exactly like their parents has lately been shown 
to be quite erroneous. Warren 2 in his studies on the “partheno- 
genetic” eggs of Aphis, and Simpson 3 in Paramoecium have 
demonstrated that individuals produced asexually from a single 
parent may differ very considerably among themselves. 
There can be little doubt, then, that the process of exogamous 
fertilisation has the effect of renewing in the gametes (partly it 
seems by a direct chemical effect) the full power of cell-division if that 
power has been lost or decreased; of mingling two lines of descent 
and so bringing about and controlling variations ; and of doubling the 
number of chromosomes (or their equivalents) in the cell. These 
effects would seem to have been produced in what was probably the 
primitive form of fertilisation, that of the fusion of free-swimming 
1 The Chromosomes in Heredity, Biol. Bull. IV. (1903), p. 231. 
1 Proc. Roy. Soc., LXV. (1900), p. 155. 
3 BiometriUa I. (1902), p. 400. Weismann also earlier recognized 
variations in parthenogenetically developed species of Cypris. 
