EVOLUTION 205 



applicable here. Becombinations occurring at the reduc- 

 tion division can be utilized by the gametophyte as well as 

 by the sporophyte, hence there seems to be no necessity 

 for plants to change from dominant gametophytes to 

 dominant sporophytes in order to secure the greater 

 adaptability offered by sexual reproduction. Every com- 

 bination of characters possible in the sporophyte occurs 

 in the haploid condition, if we leave out of considera- 

 tion heterozygous combinations which are the interaction 

 of two members of a contrasted pair of factors and can- 

 not be fixed. Haploid combinations even have certain 

 advantages over diploid combinations in that all the char- 

 acters are expressed and offer more material to natural 

 selection. Favorable combinations have a better chance 

 of survival and unfavorable combinations are more 

 quickly eliminated. If, then, variability is not a factor 

 in the rise of the sporophyte, and if we refuse to admit 

 any value in chromosome-doubling itself, and the evidence 

 certainly does not indicate that it has any significance, the 

 only factor which remains, as far as we can see now, is the 

 vigor derived from hybridization. Heterosis, of course, 

 can operate only in the sporophyte. In the lower plants 

 where the sporophyte is less important in the life cycle, 

 heterosis would be of value only in spore formation. Later 

 as the sporophyte became of more consequence, heterosis 

 would have had more and more value ; and it may well be 

 that it had considerable to do with this revolution in 

 plant life. 



If sexual reproduction is so useful that it has been 

 adopted as the principal means of reproduction at a 

 sacrifice of speed of multiplication and economy of ma- 

 terial, why, then, has it been given up by the many species 



