262 THE PRESENT EVOLUTION OF MAN 



with many others which have suffered less or not at all. If disease 

 produces any germinal change, then, no matter how small and im- 

 perceptible the difference between one generation and the next, or 

 how confused with spontaneous variations, the constant accentua- 

 tion of the alteration during hundreds, perhaps thousands, of 

 generations, must make it at last manifest and unmistakable. 

 The multitude of races and their diseases is so great that we are 

 able to make endless comparisons and so reduce the chance of 

 error to a minimum. The materials are so familiar that every man 

 may verify them for himself; whereby that grave source of error 

 and doubt, the ' personal equation ' of the observer, is eliminated. 



434. Manifestly, we are dealing now with a problem of the first 

 importance, the question of the causation of variations, which, 

 both from a theoretical and a practical standpoint, is the central 

 problem of heredity. All theories of racial adaptation and 

 degeneration, of the influence of environment, of likenesses and 

 differences between parents and offspring, of plant and animal 

 breeding, of far-reaching social effort depend on it. Disease affords 

 material for solving the problem more conclusively than anything 

 else to be met with in nature. The reader must, therefore, consider 

 carefully what theory of the causation of variations fits in with the 

 known changes undergone by races that have been in contact with 

 disease. In other words, he must make a rigorous deductive infer- 

 ence of consequences, and then carefully compare these con- 

 sequences to reality. 



435. The facts are decisive. Nearly all human races have been 

 exposed to disease for thousands of years, and in no instance is 

 there to be found an iota of evidence that any race has, as a con- 

 sequence, become degenerate, or that any race has transmuted 

 acquired into inborn immunity. On the contrary, not only is 

 every race resistant to every prevalent and lethal disease precisely 

 in proportion to its past experience of it, but the resisting power is 

 such that it can have been evolved only through the Natural 

 Selection of the spontaneous variations of a germ-plasm that was 

 insusceptible to the direct action of the environment, and did not 

 transmute acquirements into innate characters. Without excep- 

 tion, those races which have been exposed to diseases (e.g. tuber- 

 culosis), against which immunity cannot be acquired by the 

 individual, have tended to become, not enfeebled, but innately 

 capable of resisting infection ; whereas, races which have been 

 exposed to diseases against which immunity may be acquired 

 (e.g. measles), have evolved nothing other than an increased 



