184 OUTLINES OF EVOLUTIONARY BIOLOGY 



without being taught shows that the number may, in some cases 

 at any rate, be very large. Speech, however, is a comparatively 

 recent acquisition in the human race and many of the higher 

 animals perform actions without being taught which their ancestors 

 may very well have originally learnt to perform by observation 

 or experience. It would be extremely difficult to explain the 

 nest-building habits of birds, and other so-called " instincts," 

 except as having been originally acquired in this way in the 

 life-time of the individual. 



On the whole, then, the available evidence seems to indicate that 

 suddenly and exceptionally acquired characters, such as mutilations, 

 are occasionally but only rarely inherited to such an extent as to be 

 recognizable, while, on the other hand, characters which are due to 

 the continued action of some external stimulus, extending perhaps 

 over many generations, in the long run become so firmly impressed 

 upon the organism that they affect the germ cells as well as the 

 somatic cells and thus become truly blastogenic. 



We must look upon the germ cells as highly conservative 

 bodies, possessed of great inertia, which can only be induced very 

 slowly in normal circumstances, or by some sudden revolution, 

 of the nature of which we know nothing, in abnormal circum- 

 stances, to alter their constitution. This conservatism is no 

 doubt of great value as a check upon the inheritance of accidental 

 and unfavourable modifications. The adaptation of the individual 

 to its particular environment is to a large extent provided for by 

 somatogenic modifications which arise in its own life-time. What 

 suits one individual, however, may not suit its successors, which 

 may have to live under more or less widely different conditions, 

 and for progressive evolution what is needed is an average 

 adaptation of the race to an average environment. Hence the 

 germ plasm in its evolution lags behind the soma and is only 

 very slowly impressed with those characters which experience 

 has shown to be of permanent value to the race. It is a kind of 

 second chamber which acts as a useful check upon too radical 

 tendencies. 



That the germ plasm is to some extent capable of modification by 

 environmental stimuli, however, is clearly shown by the experiments 

 of Tower referred to in Chapter XL, but it is extremely difficult to 

 see how such stimuli as he mentions could affect it except by 

 acting primarily upon the soma in which the germ cells are 

 enclosed. Then the soma might be modified in response to the 



