22 THE PRINCIPLES OF HEREDITY 



under expert management. Why is it possible to increase the 

 virulence of bacilli only by passage through a living host 

 in which their enemies the phagocytes are present, and not 

 by passage through non-living media from which they are 

 absent ? It is perhaps conceivable that the transmission of 

 acquirements may cause the weakening of a toxin by bringing 

 about defects in the producing apparatus; but to me, at 

 least, it seems wildly improbable that such a process can 

 cause the strengthening of a toxin. Moreover, we shall see 

 presently that there are cogent reasons for believing that 

 modifications made under the influence of use and disuse 

 are limited to organisms immensely higher in the scale than 

 the microbes of disease. 



36. We may conclude therefore that there is as yet little 

 or no evidence to justify the belief that acquirements are 

 transmissible among unicellular forms. We are not, in- 

 deed, in a position to declare they are never transmitted. 

 The matter is still sub judice. Presumably it might be 

 settled by the success or failure of attempts to produce 

 racial change so rapidly that the influence of Natural Selec- 

 tion could reasonably be held as excluded. But already, as 

 regards many important characters of unicellular forms, we 

 are in a position to declare that it is highly improbable they 

 can have arisen under any influence save only that of Natural 

 Selection. All size and complexity is relative. Medical 

 men have realized how little and simple these organisms are ; 

 but they have not realized how large and complex they are, 

 how far removed from the beginnings of life. 



37. We may now turn to multicellular plants and animals. 

 As regards them at any rate we are in possession of an 

 immense mass of evidence. After a unicellular organism 

 conjugates it divides and re-divides many times, and the 

 resulting millions or billions of cells separate, each cell 

 1 'ganging its ain gait." A fertilized ovum also divides and 

 re-divides many times, but the resulting cells adhere together 

 and form a cell-community, in which to the germ-cells, as to 

 the queens and drones of a hive, is delegated the function 

 of continuing the race. Now, when a unicellular organism 

 makes an acquirement, it is supposed to bequeath it to its 

 own offspring and descendants. A priori transmission here 

 seems probable ; " the child is a part and usually a half of 

 its parent." But the case as regards multicellular organisms 

 is very different. One set of cells, the somatic cells, acquire 

 modifications ; the descendants of quite another set of cells, 

 the germs, are supposed to reproduce them. It is as though 



