10 HEREDITY 



where the new individual has but one parent, as 

 in cases of what is called parthenogenesis, it is not 

 an exact replica of that parent ; and the same is true 

 even where sex is unknown and reproduction is ob- 

 served in its simplest form. This difference between 

 the begetting and the begotten might conceivably be 

 due to the fact that the surrounding circumstances 

 or environment of the one were not precisely iden- 

 tical with those of the other. But even when the 

 environment is the same for both, this difference is 

 still found, and we express it by the general term, 

 Variation. 



This fact, also, is one of constant experience ; but 

 our opinion of it is not that it " stands to reason," but 

 rather that it is irrational. We incline to think that 

 heredity is the " law," variation the " accident," or 

 the exception to the law. Strictly speaking, we 

 fancy, there "ought to be" no such thing as varia- 

 tion ; but of course it is unreasonable to expect that 

 heredity shall always be perfect, and " something or 

 other " is doubtless often apt to mar its accuracy, thus 

 leading to that anomalous (or, in English, lawless) 

 occurrence which we call variation. 



On second thoughts, however, we see that our 

 notions are quite unphilosophic. Causation is non- 

 versal ; and variations cannot be regarded as " flukes " 

 or flaws in heredity, but must be subject to laws of 

 their own. It will be part of our business, then, in 

 this book to ask ourselves whether variation, as well 

 as heredity, does not " stand to reason," in the sense 

 that it is capable of a rational explanation. 



In our study of these matters, we might save our- 

 selves much trouble by the adoption of a metaphorical 



