vii HAPLOID AND DIPLOID PHASES 215 



91, A, is phylogenetically the oldest, from which the other conditions have 

 been derived. While all such discussions are of course highly speculative, 

 it is certainly probable that the earliest organisms, before sexual repro- 

 duction was evolved, were haploid. It is scarcely possible to avoid the 

 conclusion that the condition in a diploid organism with its duplex set 

 of all hereditary factors is a secondary one and the direct consequence 

 of the introduction of syngamy, while the function of meiosis is to bring 

 back the organism to its original haploid condition. 



At present we cannot say for certain whether any organisms exist 

 in which sexual reproduction has not yet been evolved, and which 

 therefore have no diploid stage ; obviously it would be a difficult matter 

 to identify such organisms, since in the absence of an alternation of 

 diploid and haploid generations there would be no certain criterion for 

 deciding whether the number of chromosomes (in the unlikely case that 

 this could be determined in such a primitive organism) were the haploid 

 or diploid number. Moreover, it would be necessary to prove a negative 

 namely, the non-occurrence of occasional syngamy and diploid stages. 



Nevertheless it is possible to cite at least one case of a life cycle which 

 has secondarily become completely haploid namely, the fern Lastraea 

 pseudo-mas, var. cristata. In this fern the sporophyte and the game- 

 tophyte have the same number of chromosomes, and this (as can be 

 determined by comparison with its near allies) is the haploid number. 

 One generation passes into the other without syngamy in the one case 

 or meiosis in the other. This is a feature which has obviously been 

 acquired but recently from an evolutionary point of view, and therefore 

 it is shown as outside of the series in Fig. 91. The converse case, where 

 organisms have entirely eliminated the haploid phase from their life 

 histories, can be illustrated by many cases of parthenogenesis and asexual 

 reproduction in animals and plants. 



It is interesting to note that the haploid and diploid conditions in 

 plants are not necessarily associated with a particular type of structure. 

 For instance, in ferns, the above-mentioned Lastraea pseudo-mas, var. 

 cristata, has a haploid sporophyte of the usual type of structure, though 

 in ferns generally the sporophyte is diploid and the haploid condition 

 is associated only with the prothalloid type of structure. The converse 

 is the case with another fern, Athyrium felix-foemina, in which the 

 prothallus is diploid as well as the sporophyte. Analogous cases 

 in animals are the haploid individuals developing in certain cases of 

 artificial parthenogenesis and the males of many Hymenoptera. 



For a general discussion of the problems of alternation of generations 

 in plants, the reader is referred to Bower (1919). 



