226 FUNDAMENTALS OF CYTOLOGY 



Conclusions. — We may summarize here the contributions that 

 cytology has made to our understanding of hybridity. 



It has revealed much of the physical basis of peculiar modes of geneti- 

 cal behavior and grades of sexual sterility in known hybrids. 



It has furnished a means of detecting hybridity and probable origin 

 in some organisms not giving other clear evidence. For example, it has 

 shown why some plants that breed true must be regarded as hybrids. 



It has shown that a significant association often exists between 

 hybridity, heteroploidy, apomixis, and certain types of mutation. 



It has afforded a partial explanation of how unstable hybrids may, 

 after some generations, yield new stable and fertile types. 



It has revealed in chromosomal alterations an inner evolution which 

 plays some role in the evolution of external diversity among organisms. 



It has offered suggestions as to modes of procedure in attempts to 

 produce new fertile types through hybridization. 



It has devised artificial methods of conferring fertility, with tetra- 

 ploidy, upon desirable but sterile hybrids. 



It has revealed a number of variants of the fundamental physical 

 basis of hybridit}^ among organisms. 



Finally, it has given us a far better conception of the cytogenetical 

 history of present organisms, enabling us better to predict what their 

 future may possibly be. Change is the rule in natvire, and cytogenetics 

 has shown us some of its inner causes. 



