1905.] Study of Conjugation in Paramecium. 



383 



are of considerable significance in connection with current views regarding 

 the theoretical relation of the Protozoa to evolutionary problems. The results 

 show clearly that, so far as the present material is concerned at least, 

 (a) there is a differentiated " conjugant type " of Paramecium ; and (b) that 

 this "conjugant type" is relatively fixed and constant under varying 

 environmental conditions, as compared with the type of the general 

 population in fission generations. If the individual Paramrecia of a given 

 race must conform to a definite and relatively fixed morphological type 

 every time they conjugate, what they may acquire during fission generations 

 is clearly of no particular account to the evolutionary history of the race in 

 the long run. Real evolutionary progress will depend on changes in the 

 " conjugant type," just as in the Metazoa real evolutionary progress depends 

 on changes in the germ cells rather than in the soma. 



Attention is called to the significance which a high degree of homogamy, 

 whenever it exists, must have as a factor in divergent evolution. A high 

 degree of homogamy furnishes at once the means whereby a new variety 

 or species may be differentiated from a parent species, though both continue 

 to live together in the same area or environment. Because, if like mates 

 with like, with any considerable regularity or uniformity, whenever 

 individuals appear showing deviations from the general population, those 

 which have on the whole like deviations will tend to mate together rather 

 than with individuals unlike themselves belonging to the parent stock. 

 Hence, the variations will not be " swamped by intercrossing " before natural 

 selection has an opportunity to act on them. Hitherto, the existence of a 

 high degree of homogamy has not been demonstrated quantitatively in any 

 organism living in a state of nature. The results of the present work show 

 that not only is such a degree of homogamy possible under natural condi- 

 tions, but that it actually exists in one case of what must be considered 

 as the simplest forerunner of the sexual processes of higher organisms, 

 namely, the conjugation of a protozoon. 



VOL. LXXVJI. — B. 



