156 THE EVOLUTION OF MAN 



builder, it is altogether probable that the same is true of intel- 

 lectual and social evolution. 



Mutations and Their Causes. Regarding the causes of mu- 

 tations we know very little, but it seems logically necessary to 

 assume that these causes are both intrinsic in the germ-plasm 

 and extrinsic in the environment; or, in the language of Dar- 

 win, "Although every variation is either directly or indirectly 

 caused by some change in the surrounding conditions, we must 

 never forget that the nature of the organization acted upon 

 essentially governs the results." The same outer conditions 

 acting upon different species or individuals produce different 

 results and the possibilities of evolution are always limited by 

 the organization of the germ-plasm. Some conceivable muta- 

 tions do not and cannot appear because of these limitations. 

 "Whales never produce feathers, nor birds whalebone," said 

 Huxley; and probably no one ever really saw a green horse or 

 a purple cow. But, although mutations cannot take place in 

 all conceivable directions, there is no justification, in recent 

 experimental work, for extreme views of orthogenesis which 

 regard mutations as taking place in only a single direction. 

 Furthermore, an extensive study of mutations shows that they 

 are as frequently injurious as beneficial; indeed they are rarely 

 as well fitted for existence as the stock from which they come. 



It is highly probable that mutations take place in response 

 to changes in environment, but it is necessary to remember 

 that the environment of the germ-plasm is not merely the outer 

 world but also the inner environment of the body organs and 

 fluids and cells, and the innermost environment of the cyto- 

 plasmic and nuclear substances which surround the inheritance 

 factors or genes. 



Inheritance of Acquired Characters. But in spite of the fact 

 that mutations probably occur in response to changes in the 

 inner or outer environment, it is practically certain that modi- 

 fications or fluctuations of developed organisms are not trans- 



