136 AMPHIMIXIS OR ESSENTIAL MEANING OF [XII. 



all parts and organs can be ensured in every case. The 

 extraordinary superfluity of individuals in each generation is 

 indispensable for that intense selective process which must 

 have operated without ceasing if it is to afford the explanation 

 of universal adaptation. And if among the thousands of germs, 

 which sooner or later succumb in the struggle for existence, 

 there were alwa3^s a hundred which contained the same com- 

 bination of individual characters, // is clear that this number 

 would not count for more than one, as material for natural 

 selection. It is just because each fertilized germ, and the 

 individual arising from it, are different from others as regards 

 the combination of characters, that the completeness of adapta- 

 tion is rendered possible. It follows from this arrangement 

 that the highest possible number of combinations of germ- 

 plasm are offered for the operation of natural selection. 



It must furthermore be borne in mind that the full number 

 of possible combinations, which is mathematically calculable, 

 is, in practice, very far from being attained. We must assume 

 in the calculation that the nuclear rods possess a limitless 

 power of combination ; but this is neither proved, nor is it 

 probable. We are on the safe side in assuming that certain 

 combinations are formed more readily than others, and are 

 for this reason of more frequent occurrence. And it must 

 not be forgotten that identical ancestral units (ids) and identical 

 idants may be present in the germ-plasm. Widely different ids 

 are not contained in every individual of a species, and perhaps 

 never occur in the same individual. In many cases the two 

 parents are in some degree blood relations, and would con- 

 tain similar or similarly composed idants. Although direct 

 observation can tell us nothing on this point, it can still be 

 shown that identical idants may be found in one and the same 

 nucleus. This is proved by the doubling of the rods which 

 takes place before the ' reducing division,' and it can be inferred 

 with equal certainty from other conditions. 



The two idants which arise in the mother-cell of the ovum, 

 by the longitudinal splitting of a single one, must contain similar 

 combinations of ancestral units. If this were not so, it would 

 follow that each of the two daughter nuclear rods would contain 

 different ids, and hence the number of ids in each single idant 

 would necessarily be diminished by half. But this cannot be 



