300 THE THEORY OF THE GENE 



occurred about this as a new mode that overstepped the 

 previous boundary, further advances would be expected 

 to appear in the direction in which the last advance took 

 place. In other words, selection would bring about further 

 advances in the direction in which each selection had 

 taken place. 



But, as I have said, Darwin never made use of this 

 argument in favor of his selection theory, although it 

 might be claimed he did so in principle whenever he 

 found natural selection inadequate to explain a situation 

 and appealed to Lamarck's principle to carry through 

 the new advance. 



Today we regard the selection process, whether natural 

 or artificial, as capable, at most, of causing changes only 

 to the extent to which recombination of the genes already 

 present may affect a change ; or, in other words, selection 

 cannot cause a group (species) to transcend the extreme 

 variations that it naturally shows. Rigorous selection can 

 bring a population to a point where all of the individuals 

 are nearer to the extreme type shown by the original 

 population, but beyond this it cannot go. Only by the 

 occurrence of a new mutation in a gene, or by a mass- 

 change in a group of old genes, is it possible, as it now 

 appears to us, for a permanent advance — a step forward, 

 or backward — to be made. 



This conclusion is not only a logical deduction from 

 the theory of the stability of the gene, but rests on numer- 

 ous observations showing that whenever a population is 

 subjected to selection, a rather rapid change begins, but 

 quickly slows down and soon comes to a standstill at or 

 near the extreme type shown by a few individuals of the 

 original population. 



So far the problem of the stability of the gene has been 

 examined with respect to gene-contamination in the hy- 

 brid, and from the point of view of selection. The possible 



