276 Experimental Transformatio7i of Species 



cies have been hybridized are comparatively few, because 

 in many cases we find the chromosome mechanism of the 

 two hnes incompatible at one or more points. Such incom- 

 patibilities account for the frequent interspecific sterility, 

 which used to be considered the essential test of species. The 

 incompatibilities arise also, however, in the course of the ex- 

 perimental study of known mutations from a common 

 ancestry. 



The appearance of a mutation in a given direction does 

 not of itself increase the probability that further change will 

 continue in the same direction. In so far, however, as a mu- 

 tant continues to live and to reproduce itself, its constitution 

 and its capacity for producing new mutations will be a fac- 

 tor in further change. 



The facts of mutation so far observed and the facts of 

 heredity determined experimentally give us unmistakable 

 assurance that the transmutation of species is in accord with 

 the nature of living things. These facts also give us a clue 

 to the mechanism whereby living strains maintain their sta- 

 bility on the one hand, and adjust themselves, on the other 

 hand, to changing conditions in the course of ages. 



