160 The Bible of Nature 



Causes of Variations. — As to the causes of varia- 

 tions and mutations we know very little. We 

 must still repeat Darwin's words, ''Our ignorance 

 of the laws of variation is profound. Not in one 

 case out of a hundred can we pretend to assign any 

 reason why this or that part has varied." It is 

 probable that variability is, like growth, a primary 

 quality of living things, and that "breeding true" 

 has arisen secondarily as a restriction. The re- 

 lation of genetic continuity between successive 

 generations is an economical arrangement which 

 secures relative constancy amid continual flux. 

 In spite of this, however, the Proteus continually 

 asserts itself. There may be, for all we know, a 

 process of growing and varying inherent in the 

 germ-plasm, requiring only an occasional envi- 

 ronmental stimulus to keep it agoing. We must 

 remember that the germ-plasm, though marvel- 

 lously stable in its general architecture, has the 

 instability involved in great complexity. Sur- 

 rounding it there is the very complex, very varia- 

 ble, nutritive environment of the body. In the 

 processes of maturation there is an extraordinarily 

 elaborate shuffling of the cards which we call 

 chromosomes. In fertilization, at the beginning 

 ot almost every new life, we see the making of a 

 living mosaic of parental and ancestral contribu- 

 tions, and there is abundant opportunity for new 

 permutations and combinations. 



