THE PHYSICAL BASIS OF LIFE 205 



doctrine of the direct transmission of the various living sub- 

 stances employed in the make-up of the individual lands us in 

 this difficulty. The fertilised egg has all the material necessary 

 for the make-up, therefore it can, and does, develop into an 

 adult. The generative cells also possess amongst them all the 

 necessary material. Therefore amongst the earlier generations 

 of cells produced by the growth and division of the fertilised 

 ovum that cell or those cells which will form the generative 

 organs must contain all the substances. But direct experi- 

 ment contradicts this conclusion. Possibly in the very first 

 cleavage, certainly in the second cleavage of the egg, there is 

 a distribution of material amongst the two or four cells such 

 that each one lacks something in the general make-up, and there- 

 fore can, and will, grow only to an imperfect monster if isolated. 

 But one of those four incomplete cells will give rise, amongst 

 other things, to the generative organs, each cell of which, in 

 the first instance, is complete. Therefore, as we may " neither 

 confound the persons nor divide the substance," we seem to 

 be in a region of incomprehensibles. 



"Just as that normal truth to type," says Bateson, "which we 

 call heredity is in its simplest elements only an expression of 

 that qualitative symmetry characteristic of all non-differentiating 

 cell divisions, so is genetic variation the expression of a 

 qualitative asymmetry beginning in gameto-genesis [the genesis, 

 that is, of the germ cells]. Variation is a novel cell division. . . . 

 What is the cause of variation ? " Cross-breeding — that is, the 

 union of unlike germ cells — may modify the character units. 

 So, too, apparently may the long-continued absence of cross- 

 breeding. It has been noticed in the cycles of a pure strain 

 of Paramcecium that the periods of depressed vitality are also 

 periods when the directive force of heredity is weakened. The 

 individuals of successive generations show great departures 

 from the normal type, and monsters are of frequent occurrence. 

 With the lowered rate of growth, the lowered " vitality," as we 

 call it, for want of a more precise word, there is associated a 

 lowered degree of fixity of type. 



