VIII A THEORY OF HEREDITY 281 



It is quite certain that in some infective diseases the 

 contagion is spread by specific material particles. 

 This seems to be established, although it is far from 

 settled as to whether these particles are parasitic 

 organisms or portions of the diseased organism itself. 

 Mr. Darwin's pangenetic gemmules may, even if not 

 accumulated and transmitted from generation to 

 generation, be called upon to explain the solidarity of 

 the constituent cells of one organism ; they may be 

 assumed as agents of a peculiar kind of infection,^ by 

 means of which the molecular condition or force- 

 affection of one cell is communicated to others at a 

 distance in the same organism. It is difficult without 

 some such hypothesis of an active material exchange 

 of living molecules between the various cells of the 

 body, to conceive of the way in which " change is 

 propagated throughout the parental system," or a 

 modified part is to " impress some corresponding 

 modification on the structures and polarities " of dis- 

 tant units, such, for example, as those contained in 

 the mammalian ovum. 



In the human ovary no egg-cells are produced 

 after the age of two and a half years. Each of the 

 many hundred eggs there contained reposes quietly 

 in its follicle, whilst the growth and development of 

 other organs is proceeding. Then a renewed period 

 of activity for the ovary commences, but the majority 



1 It is a striking exemplification of the unity of biological science 

 that we should have to look to the pathologist for the next step in 

 this region of speculation, and that fermentations, phosphorescence, 

 fevers, and heredity, should be simultaneously studied from a common 

 point of view with psychology. 



