98 VARIABILITY 



admit for the moment that the medical inference is true for the 

 present day, and that disease and drink act on the human race as 

 the climate of India is said to act on recently imported European 

 dogs. It is not denied, indeed it is a matter of common observation, 

 that diseased or drunken parents frequently have perfectly normal 

 offspring. Here, at once, we see Natural Selection at work on the 

 germ-cells. These normal individuals, whose germ-plasm was 

 insusceptible, are those that continue the race. If, then, the germ- 

 plasm of the race is not now, after thousands of years of selection, 

 as a general rule insusceptible, it is in process of becoming so. 

 Possibly the American maize mentioned by Metzger furnishes 

 another instance of this kind of selection. 



158. The conclusion we reach, then, is that, though variations 

 may result from the direct action of the environment, such varia- 

 tions are, in effect, always injuries, and are of rare occurrence in 

 individuals who survive and have offspring. Adaptation (i.e. 

 evolution) depends almost exclusively on spontaneous variations. 

 These do not imply damage to the germ-plasm, but are products 

 of its vital activity. Occurring in vast abundance all round the 

 specific and parental means, they supply the sole material for 

 Natural Selection. 



159. We conceive the germ-plasm, then, as living and active, 

 closely adjusted to its environment, growing, dividing, varying, 

 capable of being destroyed and injured, but resisting death and 

 injury, and within limits capable of repairing damage and 

 returning to its original state as behaving exactly as a living 

 individual does. Plenty of biologists think of it as drifting like a 

 dead thing at the mercy of circumstances, incapable of making 

 vital reactions, capable of undergoing all sorts of radical alterations 

 and yet of surviving. But never yet have I met anyone holding 

 this opinion who was willing to make a rigorous deductive 

 inference of consequences, and explain how it happens that 

 races exposed to unfavourable conditions undergo, not deteriora- 

 tion, but adaptation. As well might it be maintained that if a 

 man goes continually down hill he will ultimately arrive at the 

 top. 



1 60. The problem of the causation of variations is, both 

 theoretically and practically, one of the most important of all 

 biological problems. A discussion prolonged but singularly futile, 

 and founded mainly on experiments and observations, innumerable 

 but peculiarly irrelevant, has raged about it. From such experi- 

 ments as those of Clayton on beans, men have inferred that all 



