258 THE GERM-riJ^SM 



parent is certainly always represented by one-half, but the share 

 varies even in the case of the grandparent ; in the instance just 

 given it would varv between o and 16. For the reducing di- 

 vision may, for instance, cause the sixteen paternal idants result- 

 ing from the reduction of the thirty-two originally present in 

 the sperm-cell, to contain idants derived from the grandfather 

 only, and none from the grandmother ; or, again, there might 

 be fifteen from the grandfather, and one from the grandmother, 

 or fourteen pp and two //////. or thirteen pp and three ww, and 

 so on.* This would be so, at least, if any kind of combination 

 of the idants may result from the reducing division. It may 

 perhaps not be the case absolutely, but the capriciousness with 

 which reversion to a grandparent may occur nevertheless in- 

 dicates that a considerable latitude exists with regard to this 

 combination. 



In passing back to the third, fourth, and fifth generations, we 

 cannot in the least determine, a priori^ to what extent an indi- 

 vidual ancestor of the animal in question is still represented in 

 the germ-plasm of a germ-cell ; we can only state the maximum 

 which might be possible in the most favourable case. In the 

 above-mentioned instance, an ancestor of the third generation 

 jjiig/it still be represented by sixteen idants, for the sixteen 

 idants which this ancestor furnished for the purposes of amphi- 

 mixis in the second generation might, all in fact, possibly have 

 passed into one germ-cell in the process of reducing division in 

 this generation, and the same, again, might have occurred in the 

 first generation. Such a case can only be of rare occurrence, 

 but it apparently accounts for the instances of reversion in Man 

 to ancestors more remote than grandparents, which, though 

 rare, certainly occur occasionally. The more remote the gen- 

 eration, the greater are the chances against the entire half of 

 the total number of idants remaining together through several 

 generations in individual germ-cells, and the probability of such 

 an occurrence will very soon be reduced to zero. 



We may suppose that, as a general rule, the number of ances- 

 tral idants contained in a fertilised egg-cell becomes less in pro- 



* I shall now denote the paternal ids or idants by the letter p, the 

 maternal ones by /«, and those derived from the grandfather hy pp or 

 ptn, and from the grandmother by mp or mm, &c. The first letter in each 

 case signifies the parent, the second grandparent, the third the great- 

 grandparent, and so on. 



