32 HEREDITY 



constructed a most ingenious and plausible and 

 reasonable theory in which most of us have fully 

 believed until recent times. The theory was that 

 the sole cause of variations in all organisms save the 

 very lowest is the intermixture of two somewhat dis- 

 similar germ-plasms in the act of bi-parental repro- 

 duction. Observe that each gamete loses half its 

 chromosomes, and the new cell formed from the two 

 thus contains only a portion of the elements of each. 

 The natural supposition was that there is a germinal 

 selection of parental characters ; some are taken, 

 others left : and hence the new individual must vary 

 from either of his parents, and need by no means 

 necessarily "strike an average" between them. In 

 other words, the function of sex is the production of 

 variations ; and the known facts seem to afford a 

 ready explanation of the manner in which such 

 variations arise. 



Nevertheless, this most satisfactory and consistent 

 theory must be totally repudiated. It is not enough 

 in science, though it has alwavs sufficed in meta- 

 physics and in theology, that a theory be self- 

 consistent and logical. The contention of science is 

 that the theory must be consistent not merely with 

 itself, but also with the facts. We may remember 

 Huxley's joke as to Spencer's idea of a tragedy : " A 

 deduction killed by a fact." That is the irremediable 

 accident that has befallen Weismann's explanation 

 of the origin of variation and the function of 

 bi-parental reproduction. 



In the first place, it is found, on a priori mathe- 

 matical grounds, that the effect of amphimixis — as 

 Weismann calls the process of nuclear conjugation — 



