Vol. XVII. pp. 83-90 April 9, 1904 



PROCEEDINGS 



OF THE 



BIOLOGICAL SOCIETY OF WASHINGTON 



THE VEGETATIVE VIGOR OF HYBRIDS AND MUTA- 

 TIONS.* 



BY O. F. COOK. 



Under what has l)een termed a kinetic theory of evolution f it 

 has been held that the condition most favorable to evolutionarv 

 progre.ss is that found in natural species containing numerous 

 individuals, widely distributed and freely interbreeding. The 

 individual diversity of members of large assemblages of organ- 

 isms is greater than when interbreeding is confined to narrow 

 limits, but under persistent close breeding uniformity or " fixity " 

 of type is followed, eventually, by very pronounced and abrupt 

 variations, and by a decline of reproductive power. 



On the other side of the evolutionary highway corresponding 

 phenomena abound. Interbreeding among the normally diverse 

 members of a species in nature strengthens the organism and 

 aids in distributing variations throughout the species, but when 

 individuals from small, close-bred groups are crossed their char- 

 acters may prove antagonistic, and not to be combined or aver- 

 aged in the offsi)ring, as discovered by Mendel. When still 

 more remote types are brought together the resulting hybrids 

 are often abnormally diverse, and may have characters possessed 

 by neither of the parents. Because pronounced variations are 

 thus obtainable both by narrow inbreeding and by wide cross- 

 breeding these extreme stages have been thought to have great 



* Read before the Biological Society of Washington, November 28, 1903. 

 tScience, N. S., 13: 969, 1901 ; Popular Science Monthly, 63: 18, 1903. 



10— Pboc. Biol. Soc. Wash. Vol. XVII, 1904 (83) 



