INTRODUCTrON. H 



That these convei-ginu- (hita pointed to something- of fundamental 

 evolutionary .si^'iiiMeance has ))eenc'onhdentl3' believed since the publi- 

 cation a decade aoo of Strasburg-er's epoch-making essay entitled '' The 

 Periodic Reduction of the Number of the Chromosomes in the Life- 

 History of Living- Organisms/'" but a new evolutionary standpoint 

 •was required before the larger import of the facts could be perceived. 

 The reduction of chromosomes is indeed a striking and unique phe- 

 nomenon in the life history of organisms, and it naturally became the 

 focus of interest in the rapidly developing science of cytology, A 

 new point of view was the more necessary, however, because of an 

 unfortunate choice of terms which has undoubtedly tended to prevent 

 the perception of the true relations of the facts, as it now interferes 

 with a correct description of them. We refer to the characterization 

 of thehig-her, double-celled, spore-bearing- " generation " as "asexual," 

 Appreciating the primitive character of such structures as the pro- 

 thallus in ferns, Strasburger asserted that a new "asexual generation" 

 had been intercalated into the life history of organisms. It is now 

 perceived that for cytological purposes this is not the whole truth, 

 and that for evolutionary purposes it is not true at all. The new 

 " generation" was not merely intercalated into the life history of the 

 organism; it was intercalated into \hQ sex aal process . It is, therefore, 

 not asexual, but sexual, and in a higher degree than the so-called sex- 

 ual generation. The latter bears, it is true, the cells which conjugate, 

 but the former is produced during the actual process of conjugation. 

 Organic perfection has been attained, not through the development of 

 an "asexual generation," but by the lengthening out of the sexual 

 process itself; not by abandoning or avoiding sexuality, but directlv 

 by means of it. 



Among the lower plants the single cell formed by conjugation accom- 

 plishes in a brief space of time all the cytological processes which in 

 the higher plants come l)etween fertilization and chromosome reduc- 

 tion. Sexual fusion is immediate and complete, and takes place during 

 a brief period of interruption of the growth and su])divisi()n of vegeta- 

 tive cells. If the vegetative fern prothallus is to be termed sexual 

 because it produces antheridia and archegonia, the sporophyte is sex- 

 ual to the second degree, for it is built of conjugating cells, contain- 

 ing, until synapsis and the subsequent reducing- divisions, a doul)le 

 number of chromosomes, the parental chromatin elements being still 

 unfused. However important chromosome reduction may be, it is, 

 after all, only a corollary or sequel of the douhliiuj conjugation. It 

 was not the reducing division, ])ut tJie long postponement of the reduc- 

 tion division ^\\\\K'\\ permitted higher t^'pesof organisms to be developed 

 by means of double, sexual cells. 



A special evolutionary significance was ascribed to the chromosome 



•' Strasbiirjrcr, Ivlw aid, \nii:.is,.l I'.dtiuiv, 1S94, S: 2S1--.S1(>. 



