I '. 1 RL 1 TION IN PEDIGBEE-C 'ULTl'llES 



2 1 9 



The use of seeds of the desirable form, either from the original or from 

 any of its derivatives, might give play to all the complex activity of 

 the splitting of hybrids and of the free play of fluctuating variability. 

 During the course of study of the fluctuating variability of a species, 

 by means of successive generations grown from purely fertilized seeds, 

 the observer may be so fortunate as to encounter individuals which do 

 not fit into his series by reason of the possession of some quality not 

 visible in the greater number of the progeny, or by the lack of such 

 qualities. Thus in a progeny of red-flowered plants one may he en- 

 countered which has white bloom, or an individual with laciniate leaves 

 may come in a pure progeny the remainder of which has entire leaves, 



Fin 4. Opunlia fulgida and O mammillata growing in Contiguity in the 

 Vicinity of Desert Laboratory, Tucson, Arizona. 



or a wholly glabrous specimen may be in a hairy progeny, or an indi- 

 vidual may depart from the progeny in all these particulars. In either 

 case it is apparent that the variability here is not one of the modifica- 

 tion of a character, but by the total accession or loss of a character, and 

 the variability is therefore a discontinuous one. In the progenies in 

 which such variants have been seen hitherto, they form a proportion 

 never larger than six or eight per cent, of the whole number and gen- 

 erally in much less quantity. If such mutants or saltatory variants be 

 found, they should be closely followed, as they may furnish the observer 

 with facts of the greatest value. Care should be taken to secure purely 

 fertilized seeds and a minute anatomical examination should be made 

 of the entire plant, with special respect to the characters in which it 

 appears to differ from the mass of the fluctuating variants around the 

 type. 



