THE ORIGIN AND DEVELOPMENT OF ORGANISMS 335 



which he lives, would affect his body during life, but would not 

 be expected to affect his progeny; and acquired characters 

 should not be transmitted by heredity. This conclusion, quite 

 at variance with the beliefs of twenty-five years ago, has been 

 subjected to long and exhaustive study, as a result of which 

 the belief in the inheritance of acquired characters has gradually 

 disappeared. The conclusion has been vigorously disputed, and, 

 since the advancement of Weismann's theory of heredity, the 

 most active search has been made for proof or disproof of the 

 idea that acquired characters can be inherited. While many 

 apparent instances of such inheritance are easily found, they all 

 prove illusive when carefully studied, and biologists have prac- 

 tically agreed that there is no good evidence that acquired char- 

 acters can be transmitted to subsequent generations. While the 

 possibility of the inheritance of acquired characters cannot yet 

 be positively denied, it is quite generally believed to-day that 

 it does not occur. This conclusion has far-reaching results, for 

 it entirely changes our conception of the relation of parent to 

 offspring. 



Heredity and the Union of Sex Cells. We are in a position 

 now to appreciate a little more fully the significance of the 

 factor of the union of sex cells in sexual reproduction. Thus 

 far heredity has been spoken of as associated with eggs only. 

 A succession of similar types in successive generations can be 

 explained as due to a division and transmission of a continuous 

 germ plasm. But the result of such a process would seem to 

 produce a series of like individuals, without any variation in 

 successive generations. Successive generations are, however, 

 not alike. Indeed, the development of animals and plants is 

 dependent upon the fact that successive generations show more 

 or less divergence from the original type. It is here that we 

 see one reason for sexual reproduction. 



In Chapter XII we have noticed that the reallj- significant 

 feature of the union of the egg and the sperm lies in the fact 

 that each of these reproductive cells throws away part of its 



