ALTERNATION OF GENERATIONS 491 



A strong antithesis has been drawn between the relative faikire of 

 the gametophyte and the ultimate triumph of the sporophyte in sub- 

 aerial life. The two generations differ normally in chromosome-number. 

 This seems to suggest that a higher potentiality and initiative in varia- 

 tion lies with the diploid state. It may not be possible to lay it down 

 as a general proposition that a double number of chromosomes is an 

 index of greater power of adaptability ; but it is a significant fact 

 that the highest somatic evolution both in Animals and Plants has 

 been attained by diploid, not by haploid tissues. The sporophyte 

 is a non-sexual, or neutral generation, by nature and by origin. But 

 the steps in obliteration of the gametophyte, and in the evolution of 

 the seed, have involved the spread of the sex-difference backwards 

 in the individual life, so as to affect the antecedent neutral genera- 

 tion. This will be apparent on comparison of the hfe-cycle of a 

 homosporous Fern (Fig. 291, p. 348) with that of a Flowering Plant 

 (Fig. 244, p. 299). The final culmination of this is found in those 

 Seed-Plants which are dioecious, such as the Willow, or the Yew 

 (Fig, 245, p, 300). In these some individuals bear only staminate, 

 others only pistillate flowers, and the plants are thus ranked as " male " 

 or " female " : though in point of fact they represent the neutral 

 generation. The end result is thus seemingly a parallel between the 

 Higher Plants and the Higher Animals as regards sexuality. In both 

 the individual appears to be either " male " or " female." But this 

 similarity is superficial rather than real, for it has been attained along 

 quite distinct evolutionary channels in the two Kingdoms. In the 

 Higher Animals there is a true sex-difference between individuals, the 

 one producing male, the other female gametes. In the Flowering Plants 

 the individual is the neutral sporophyte, which does not itself produce 

 gametes. But in the course of Descent certain distinctive features 

 relating to sex have been reflected back from the sexual gametophyte 

 which it ultimately bears, and comparison shows clearly the steps 

 by which those features have been impressed upon the neutral Plant. 

 Accordingly the Flowering Plant has secured such advantages as 

 follow from sex, through its retention of the sexual generation within 

 it. Nevertheless the substantive Plant is actually the neutral generation^ 

 or Sporophyte, which is thus seen to have established its final ascendency 

 ill the Vegetation of the Land. 



