258 THE PRESENT EVOLUTION OF MAN 



It follows, if there is any truth in the theory of evolution 

 through Natural Selection, that a race much afflicted by a chronic 

 disease for example, tuberculosis should evolve inborn immunity 

 against it ; whereas, if afflicted by an acute disease for example, 

 measles it should tend to develop an inborn power of acquiring 

 immunity. Both inborn immunity and the power of acquiring 

 immunity arise in the individual through the experience his race 

 has had of disease. They are nutritional characters. But actual 

 acquired immunity arises in the individual through his own 

 personal experience of disease. So also a man's hand and his 

 power of acquiring callosities on it are due to an evolution in 

 his race which enables him to develop those characters under the 

 influence of nutrition ; but the actual callosities are due to his own 

 experience of rough work. 



429. Again, we have seen that there are several theories of 

 evolution, each with its corresponding theory of heredity. First, 

 there is the theory of an internal adaptive growth-force, which 

 supposes that variations tend, on the whole, to be adaptive. This 

 hypothesis, which is founded purely on an appeal to miracle, 

 need not be considered further. Science ought not to appeal to 

 the supernatural until, at least, it has been demonstrated that 

 natural interpretations are impossible. Second, there is the 

 Lamarckian doctrine, which supposes that characters acquired by 

 parents under the influence of use or injury, tend to be reproduced 

 by offspring under the stimulus of nutriment. Third, there is that 

 very popular theory of heredity which supposes that variations 

 commonly arise through the direct action of the environment on 

 the germ-plasm. This hypothesis of the causation of variations 

 is usually associated with the doctrine of Natural Selection, but 

 is clearly incompatible with it. Examined closely, we find that 

 it implies a theory of racial change, but not adaptive change. It 

 implies, in fact, that species drift helplessly under the influence of 

 the environment in directions which may or not be adaptive, but 

 which can very rarely be adaptive. Lastly, there is the theory 

 that the mass of variations are spontaneous, and that they occur 

 all round the specific mean, and, therefore, that the germ-plasm 

 (in so far as its hereditary tendencies are concerned) is highly 

 insusceptible to the direct action of the environment. This 

 theory, and this alone, is compatible with the doctrine of Natural 

 Selection. If only we think closely and clearly, we find also that 

 it is the only theory which fits in with the facts of adaptation. 



430. Now, which theory of heredity is borne out by human 



