No. 590] FECUNDITY IN THE DOMESTIC FOWL 



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not incessantly harp on the string that " selection caused" 

 his results, he would be on logically solid ground and 

 would receive a more respectful hearing from those who 

 place a high value upon clear thinking and sound logic in 

 scientific matters. 



Now up to this point in the argument there has been no 

 biological point involved, so far as I can see, to which any- 

 body, whether of the pure line or the selection faith can 

 take exception. Certainly I am perfectly willing to ad- 

 mit that germ-plasm changes do sometimes occur, of all 

 magnitudes from the most minute up. Further no one, 

 I take it, will deny that, having appeared, these variations 

 may be seized upon and preserved by selection. I do de- 

 sire to emphasize, however, that there is no evidence, as 

 yet, that the selection causes the variations. 



It may be objected that the postulated case is too simple 

 and leaves out of account too many factors. All this, 

 however, will not affect the logic of the case. General- 

 ized, that logic is as follows : A heritable difference be- 

 tween two individuals or races implies a difference in the 

 germ plasm. The difference in the germ plasm must have 

 made its initial appearance at a definite point of time. 

 At that time the germ plasm changed from its previous 

 condition. The cause of that change can not be conceived 

 to be the selection for breeding purposes of the parents 

 bearing the unchanged germ plasm. To assert that the 

 new variation is a result of amphimixis due to mating un- 

 like parents would be, in the present state of genetic 

 knowledge, a ridiculous begging of the question, because, 

 in the first place, by hypothesis in any selection experi- 

 ment individuals genetically as nearly alike as possible 

 are always mated together, and in the second place, as se- 

 lection continues homozygosity automatically increases. 



The whole fact of the matter is that the assertion that 

 selection per se causes changes in the germ plasm, is a 

 wholly new addition to the classic Darwinian selection 

 theory, tacked on quite inadvertently, I believe, by some 

 °f the modern exponents of that theory. Darwin never 



