EFFECTS OF AMPHIMIXIS ON THE GERM-PLASM 237 



ids must have been united into idants, which were all equal in 

 value, their number, as well as that of the ids, remaining the 

 same in subsequent generations. When sexual reproduction 

 first arose, the same number of idants from both parents became 

 enclosed in one nucleus, the total number of idants and mass 

 of germ-plasm of which were thereby doubled. This may have 

 been of no disadvantage if it occurred once only, but as the 

 process was repeated, an arrangement for preventing the germ- 

 plasm from increasing to an unlimited extent became necessary 

 each time amphimixis took place. 



Were the germ-plasm an unorganised, or even a perfectly 

 homogeneous substance with no internal differentiation, — i.e.^ 

 were it not composed of units of different orders, — its doubling 

 every time amphimixis occurred might have been prevented 

 simply by a limitation of its growth in each germ-cell, so that the 

 latter would contain only half the mass of germ-plasm formerly 

 present. But as soon as the germ-plasm came to consist of a 

 definite number of units, a diminution of the latter could not 

 result from a mere limitation as regards growth, for their num- 

 ber would nevertheless remain the same. This result could 

 only be attained by the appearance of a process by means of 

 which the mi)nber of units was reduced to half, and we have 

 seen that such a process occurs in the form of the remarkable 

 ' reducing divisions ' already described. 



It is not difficult to ascertain what changes must result in the 

 composition of the germ-plasm by the combination of this proc- 

 ess with continued amphimixis. 



Let us suppose that before the introduction of the latter process 

 the germ-plasm of a species consisted of sixteen idants. When 

 amphimixis, accompanied by the ' reducing division,' occurred 

 for the first time, eight paternal idants A would unite with eight 

 maternal idants B in the fertilised egg-cell to form the segmen- 

 tation nucleus. In consequence of the reducing division, each 

 of the germ-cells of the next generation would contain a com- 

 bination of the idants A and B, — e.f^., 4 A -f 4 B. These 

 would again unite in the next amphimixis with eight idants — 

 e.g., 4 C + 4 D — in the germ-cell of another individual with 

 different hereditary tendencies ; and the ontogeny of the third 

 sexual generation would therefore be controlled by a germ- 

 plasm composed of the idants 4A + 4B 4-4C + 4D. Let 

 us assume, for the sake of simplicity, that the reduction always 



