Genetics and Evolution 275 



that the patterns result from combinations of factors in- 

 herited in MendeHan fashion. Incidentally, field studies have 

 rendered extremely improbable the notion that the patterns 

 have any great significance in giving the species a protec- 

 tive advantage. 



Experiments have also been extended to wild animals 

 and plants so that we may say with certainty that mutations 

 presenting heritable traits appear in all species, and that the 

 Mendelian formula of inheritance is a universal phenomenon. 



Genetics and Evolution 



The experimental study of heredity^ which Mendel 

 considered essential to a clear understanding of the historical 

 process of evolution, has failed to reveal a single instance of 

 the transformation of one familiar species into another. It 

 has revealed, however, first of all the need for modifying 

 our traditional conception of species. We see from these 

 experiments that the species itself is a composite of several 

 to very many hereditary strains. We see that the species 

 is subject to change in two ways. (i) The constituent 

 hereditary factors may recombine in numberless ways. (2) 

 New hereditary capacities appear from time to time and 

 show themselves in more or less extensive modification of 

 the specific type. 



There have been described distinct species — " good " 

 species from the classifier's point of view — which turned 

 out upon examination to be mutants from other strains. If 

 the origin of such mutants had not been observed under 

 controlled conditions, we should not have suspected that 

 one species does actually give rise to another by a sudden 

 pimp. Genetic analysis (that is, experiments in hybridizing 

 and segregation) has enabled us to distinguish the sudden 

 appearance of such a new strain from the recombination of 

 characters through hybridization. Dozens of such new 

 species have arisen and they behave in every respect as do 

 natural species. Experiments in which distinct natural spe- 



