22 INTROGRESSIVE HYBRIDIZATION 



entific possibility in the Gramineae, the sugar-cane industry 

 was producing them on a commercial scale in its breeding 

 fields. If one will but leaf through such a comprehensive 

 catalogue of horticultural plant material as Rehder's Manual 

 of Cultivated Trees and Shrubs, he will gain some idea of the 

 number of interspecific and intergeneric crosses that have 

 been achieved. Unfortunately, such a compendium gives a 

 very incomplete picture. It says nothing at all about the 

 even larger number of crosses that were attempted and did 

 not succeed. 



A modern summary of the evidence of hybridization is 

 badly needed. One was last brought together by Focke 

 (1881) in his classical Die Pflanzen Mischlinge. His general 

 conclusions would find even stronger support from the evi- 

 dence that has accumulated since his day. ''Der Grad der 

 morphologischen und der physiologischen Verschiedenheit 

 entsprechen einander haufig ziemlich genau, doch gibt es 

 auch Beispiele, in denen dies durchaus, nicht der Fall ist." 

 (The degree of morphological difference is usually closely 

 parallel to that of the physiological difference, yet there are 

 examples in which this is certainly not the case.) 



To summarize: The production of hybrids fertile enough 

 to lead to gene exchange is in general common within species, 

 less common between closely related species, and rare (but 

 by no means unknown) between entities that by all other 

 criteria are distinct genera. In a very few cases hybrids have 

 been produced between genera not even closely related. 

 Only among the plants do we have enough of both positive 

 and negative evidence to generalize upon this point. There 

 are some preliminary indications (fish, tree frogs, cattle 

 relatives) that similar wide crosses may be found to be as 

 common among the vertebrates, when as high a proportion 

 of such possibilities have been experimentally attempted. 



Since the times of the early hybridizers it has been known 

 that, though many interspecific hybridizations give similar 

 results, there were a considerable number of exceptional 

 cases, such as true-breeding hybrids, segregating first-gen-^ 



