REPRODUCTION 279 



average of the species. The statistical study of variation, * 

 now so much the fashion among biologists, shows how wide 

 the range of variation is. The inevitable result of combin- 

 ing two different things is the production of a mean or 

 balance. In this balancing or equalizing (^usg-ieicA), Stras- 

 burgerf sees the essential character of sexual reproduction 

 and its fundamental advantage over all other modes of 

 reproduction. 



In certain species sexual reproduction seems to be abso- 

 lutely necessary to escape degeneration and extinction. 

 This has been shown to be the case among certain Infusoria 

 and Diatoms, but these are special cases in which the indi- 

 viduals by prolonged division (vegetative reproduction), 

 incompletely offset by growth, have become unduly small 

 and weak. By the fusion of two such individuals the nor- 

 mal amount of substance and the normal means of secur- 

 ing energy are furnished to the new individuals. Except 

 in these special cases, and in the highest animals, sexual 

 reproduction does not seem to be necessary to the mainte- 

 nance of the species in a healthy condition, in which it 

 stands a good chance to succeed in the struggle for ex- 

 istence. 



HEREDITY 



The physiologist is confronted with the problem of hered- 

 ity. He sees plants resembling one another in succeeding 

 generations and he hears discussions of the means by which 

 the characters and the tendencies of the parents are trans- 

 mitted to the offspring. What is "heredity," and what are 

 the means by which the offspring are made to possess the 

 characters and tendencies of their parents? "Heredity is the 

 biological law by which living beings tend to repeat their 

 characteristics in their descendants." J The means are of 

 two sorts — first, the continuity of substance from parent to 



* Shull, G. H. A quantitative study of variation in bracts, rays, and 

 disk florets of Aster Shortii. etc. Amer. Naturalist, vol. 36, 1902, 



t Strasburger. E. Liber Befruchtung. Bot. Zeitung, 1901. 



X See Bitter, W. E. The power and methods of heredity. University 

 Chronicle, vol. III., Berkeley, Cal. 1900. 



