Nos. 618-619] THE ROLE OF REPRODUCTION 



23:» 



cause the mechanism contributes toward mixing of germ- 

 plasms. As an example, let us take the DrosophUa type 

 of sex determination. There the sperm is of two kinds : 

 the one containing the sex chromosome and its sex-linked 

 factors, the other lacking it. The eggs are all alike, each 

 bearing the sex chromosome. It follows then that the 

 male always receives this chromosome from his mother, 

 who may have received it from either her father or 

 mother. Moreover, further variability may be derived 

 from the linkage breaks which occur always in the female. 

 This last phenomenon is hardly worthy of special men- 

 tion, however, until it is shown to be typical of such 

 reproduction. 



This short reconnaissance presents the pertinent facts 

 in the situation as they appear to the writer. A very 

 great number of interesting things connected with repro- 

 duction during the course of evolution have not been men- 

 tioned. This is because it is felt that the essential feature 

 of the role of reproduction in evolution is the persistence 

 of mechanisms in both the animal and plant kingdoms 

 which offer selective agencies the greatest amount of raw 

 material. Other phenomena are wholly secondary. 



LITERATURE CITED 



Casteel, D. B., and Phillips, E. F. 1903. Comparative Variability of 

 Drones and Workers of the Honey Bee. Biol. Bull., 6: 18-37. 



Coulter, J. M. 1914. The Evolution of Sex in Plants. Chicago, Uni- 

 versity of Chicago Press. Pp. 1-140. 



Cramer, P. J. S. 1907. Kritische Ubersicht der bekannten Falle von 



Maatsehappij der Wetenschappen. Derde VerzjLeling, Deel VI, Derde 



Davis, B. M. 1909.' Cytological Studies^n (Enothera. I. Ann. Bot. 23: 

 551-571. 



Jones, D. F. 1917. Dominance of Linked Factors as a Means of Account- 

 ing for Heterosis. Genetics, 2: 466-479. 



Kellogg, V. L. 1906. Variation in Parthen;ogenetie bisects. Science, 

 N. S., 24 : 695-699. 



Loeb, J. 1916. The Organism as a Whole. N. Y., Putnam. Pp. v-x + 

 379. 



Wright, A., Lee, A., and Pearson, K. 1907. A Cooperative Study of 

 422. 



