158 THE PRINCIPLES OF HEREDITY 



result from the play of environmental forces on the germ- 

 plasin stands on a different basis. It is inherently probable. 

 A priori it seems only reasonable to suppose that nutriment 

 or toxins or similar powerful influences persistently applied 

 must tend to cause radical changes in the germ-plasm exposed 

 to them. But when we remember that species undergo 

 evolution only under adverse conditions, and " degeneration " 

 only under beneficial conditions ; when we consider the facts 

 of adaptation, and the impossibility of adaptation occurring 

 except through the agency of unfettered selection, we are 

 forced to the conclusion that the survival of the fittest must 

 have rendered the germ-plasm nearly, if not quite, insus- 

 ceptible to external influences, so far at least as its hereditary 

 tendencies are concerned. 



263. It is often argued that, since the somatic cells of the 

 body are easily injured or benefited by influences from the 

 environment, it is absurd to suppose that the germ-cells are 

 inviolate. But this reason is founded on a double mis- 

 apprehension. It has been asserted by no one that germ- 

 cells are inviolate. It has been proved by no one that 

 somatic cells are easily altered in their hereditary tendencies. 

 On the contrary, we know that germ-cells may be readily in- 

 jured and killed, and that somatic cells are particularly stable 

 in their hereditary tendencies. Were these tendencies 

 easily changeable in somatic cells, there could be no recovery 

 from disease. During the whole period of life somatic cells 

 proliferate, and so carry on the processes of growth, repair, 

 and regeneration. Their multiplication is especially rapid 

 under adverse conditions, as when a lymphatic gland is in- 

 flamed by the presence of toxic substances. But, though 

 such a gland as a whole may be permanently altered by severe 

 and prolonged inflammation, yet the lymphatic cells if they 

 survive exhibit no hereditary change. Removal of the irritant 

 causes immediate return to the ancestral type. A month or 

 a year or twenty years after an histologist does not find a new 

 variety of cells. 1 The same is true of all the other cells of 



1 Young cells, it is true, may be altered into more mature cells, 

 thus skin cells may become horny. But no change is thereby made 

 in their hereditary tendencies. The change is one for which their 

 hereditary tendencies have prepared them. Whatever its subsequent 

 alterations the descendant of a skin or connective tissue cell is always an 

 ordinary skin or connective tissue cell to begin with. It may be that 

 a real alteration of hereditary tendencies occurs during tumour forma- 

 tion. But even so the change is merely a reversion to a remote ancestral 

 type, which external influences may call into activity but cannot create. 

 (See The Present Evolution of Man, p. 49.) 



