82 Heredity. 



by natural selection acquired distinctive properties or 

 functions, which are adapted to the conditions under 

 which it is placed. So long as these conditions remain 

 unchanged it performs its proper function as a part of 

 the body; but when, through a change in its environ- 

 ment, its function is disturbed and its conditions of life 

 become unfavorable, it throws off small particles which 

 are the germs or "gemmules" of this particular cell. 



These germs may be carried to all parts of the body. 

 They may penetrate to an ovarian ovum or to a bud, but 

 the male cell has gradually acquired, as its especial and 

 distinctive function, a peculiar power to gather and 

 store up germs. 



When- the ovum is fertilized each germ or "gemmule" 

 unites with, conjugates with, or impregnates, that 

 particle of the ovum which is destined to give rise in the 

 offspring to the cell which corresponds to the one which 

 produced the germ or gemmule; or else it unites with a 

 closely related particle, destined to give rise to a closely 

 related cell. 



When this cell becomes developed in the body of the 

 offspring it will be a hybrid, and it will therefore tend 

 to vary. 



As the ovarian ova of the offspring share by direct 

 inheritance all the properties of the fertilized ovum, the. 

 organisms to which they ultimately give rise will tend 

 to vary in the same way. 



A cell which has thus varied will continue to throw 

 off gemmules, and thus to transmit variability to the 

 corresponding part in the bodies of successive generations 

 of descendants until a favorable variation is seized upon 

 by natural selection. 



As the ovum which produced the organism thus se- 

 lected will transmit the same variation to its ovarian 



