THE ORIGIN OF VARIATIONS 105 



have relied are, I believe, not disputed, and I find it hard as I 

 think the reader will to conceive interpretations of them other 

 than those I have suggested. Germinal insusceptibility to the 

 direct action of the environment and a right degree of variability 

 are just as much adaptations, essential to the survival of the 

 species, as any of the structures and functions of the individual. 

 Presumably, therefore, they are just as much under the control of 

 Natural Selection. The limitation of variations to a particular 

 part of the germ-tract would not appear a task more difficult for 

 Natural Selection than the evolution of such wonderfully complex 

 and delicate structures as the eye and ear. 



171. We are often told that the 'cause' or 'origin' of 

 spontaneous variations is unknown. If it be meant that the 

 molecular processes in the germ-plasm which result in variations 

 are unknown, the statement is true. In that sense we do not 

 know the cause of any kind of variations, not even of those which 

 clearly result from the direct action of the environment. But in 

 another sense it is untrue as untrue as if, knowing that a certain 

 variation was caused by a certain influence in the environment, 

 we declared that the origin of it was unknown. The fertilized 

 ovum has two classes of cell-descendants, both of which vary 

 spontaneously, the somatic cells and the germ-cells. The former 

 do not vary away from type ; nevertheless they vary in a regular 

 manner from the fertilized ovum and amongst themselves, some 

 becoming skin-cells, others muscle-cells, and so on. We suppose 

 we know something very definite about the origin of these altera- 

 tions when we declare that they result from evolution and are 

 under the control of Natural Selection. Precisely the same 

 declaration may be made about the less regular but equally 

 constant variations of germ-cells. A germ-cell which has not 

 varied, has varied in that it has departed from a type which, in 

 effect, always varies. People who insist that we do not know the 

 cause of variations, and that the word 'spontaneous' is nothing 

 other than a cloak for ignorance, are unaware of the sense in which 

 that word is used, or they have not considered the evidence, 

 or they are the kind of people who insist on experimental 

 evidence and are unaware of the impossibility of devising experi- 

 ments which would furnish it. 



172. We reach, then, three principal laws of heredity three 

 summaries of facts, (a) The germ-plasm is highly insusceptible 

 to the direct action of the environment; (b) the vast majority 

 of variations all those on which evolution is founded are 



