PLANT AND ANIMAL IMPEOVEMENT 225 



crossing between distinct varieties, species or even genera 

 — is so large a topic it cannot be more than touched upon 

 here. Each particular cross presents technical problems 

 of its own. All one can say as a generality is that the 

 principle in every case is the same. Crossing brings to- 

 gether germ plasms having various attributes. These 

 attributes, the hereditary factors, recombine with regu- 

 larity and precision. They Mendelize. Prom Mendelian 

 segregation and recombination come the possibilities of 

 new and improved races. Except in those rare instances 

 when new variations previously unknown to the species 

 occur, nothing can come out of the cross that did not go 

 in. But the number of combinations possible when the 

 two parents differ by many hereditary factors is so great 

 that practically speaking many character complexes may 

 appear which have never before had the chance of showing 

 their merits or defects. In them lie our hopes. 



It was noted earlier that many species crosses are par- 

 tially sterile, that there is often a degeneration of many 

 of the germ cells and embryos, and that certain extreme 

 types are thereby produced more frequently than is usu- 

 ally to be expected. The extreme variability induced by 

 such wide crossing offers the best field in which to look for 

 the beginnings of new and valuable types of animals and 

 plants. This is not a theory; it is a general fact born of 

 long experience, for when we look into the origin of many 

 of our most valuable domesticated animals and plants we 

 find unmistakable evidence of their hybrid ancestry. 



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