I 



218 GENETICS AND EUGENICS 



themselves. The genetic changes must have occurred some- 

 time if related species really had a common origin as we, 

 under the Darwinian theory, suppose. 



Moreover, a cultivated plant, regularly self-fertilized, the 

 sweet pea, whose historic origin from a single wild species is 

 known, exists today in hundreds of true-breeding varieties 

 differing one from another in genetic constitution. All these 

 genetic changes have occurred within a few centuries and in 

 most if not in all cases within what were at the time prob- 

 ably *'pure lines." 



A common answer to the question proposed is that genetic 

 changes in pure lines are comparatively rare. Rare in com- 

 parison with what.^ With the genetic variations already 

 existing in the same species. But the latter are accumula- 

 tions of the genetic changes of centuries, or in the case of 

 cultivated wheat, of thousands of years. Is it surprising that 

 in comparison with such accumulations of variations, the 

 variations observed contemporaneously to occur in pure 

 lines are relatively few? Practically, it would be, as has 

 often been said, a '* waste of time" to look for the occurrence 

 of favorable genetic changes within pure lines of self -fertiliz- 

 ing plants, so long as a wealth of untested varieties exists 

 ready made in every commercial variety of such crop, and 

 an even greater number of new varieties may be created by 

 crossing the best existing varieties. But this is not to be 

 regarded as evidence that genetic changes have not come 

 about in the past exactly as they are coming about today, 

 within lines pure or otherwise. 



