SPONTANEOUS VARIATIONS 45 



he formulated an intelligible hypothesis of heredity, which 

 seemed to surmount some of the difficulties and explain a 

 moiety of the facts of evolution. But the hypothesis we are 

 considering surmounts none of the difficulties. It explains 

 none of the facts. It renders an intelligible theory of evolu- 

 tion impossible. It is held as a rule by the followers of 

 Darwin : apparently they do not perceive that it is even less 

 in accord with their chosen faith than the teachings of 

 Lamarck which they so strenuously reject. If variations are 

 commonly caused by the action of the environment the 

 theory of Natural Selection is certainly false. 



77. If confirmation were wanted of the truth that the 

 environment has little or no influence in the causation of 

 variations, we may find it in the fact that species, instead 

 of progressing, degenerate in all their parts when the 

 environment is entirely favourable, that is when there is entire 

 cessation of selection. Thus internal parasites tend to 

 become mere bags of eggs when only the reproductive 

 functions are selected. So also, no matter how favourable 

 the environment, choice breeds of plants and animals tend to 

 degenerate when no longer selected with care. If any par- 

 ticular structure (e. g. the limbs of the ancestors of snakes) 

 become useless (i. e. not subjected to selection), they also 

 degenerate. Evolution occurs only when the conditions are 

 unfavourable to the individual, when there is a need for 

 specific adaptation. Thus negroes, presumably by the weeding 

 out of the unfit, have undergone great evolution against 

 malaria. Unless, then, we pin our faith to a kind of topsy- 

 turvy Lamarckian doctrine, unless we suppose that influences, 

 which injure parents, cause variations of a directly opposite 

 kind in offspring, it is evident that the facts of evolution tell 

 decisively against the hypothesis that the environment is 

 commonly the direct cause of variations. 



78. All species must be in close adaptation to their 

 environment or perish ; but no species can remain adapted to 

 a constant environment, or undergo adaptive change in a 

 changing environment unless it is highly insusceptible to the 

 direct action of the forces to which it is exposed. To secure 

 adaptation the members of a species must vary spontaneously 

 in every direction about the specific mean, and so present 

 material for the work of Natural Selection. Suppose, now, 

 there existed a species the germ-plasm of which was to some 

 degree susceptible to direct alteration. Then, following the 

 rule, the germ-plasm in some germ- cells would vary so as to 

 be more susceptible than the germ-plasm in other germ-cells. 



