BIOTYPES AND HYBRIDS. 7 



presentation of my studies on Bursa before the American Association for 

 the Advancement of Science (December, 1906), I \vas pretty sure that I 

 had demonstrated the existence of 11 biotypes, but the disappearance of one 

 or two of these since, led me to reduce the number to 4, as published in the 

 extract from that paper (Shull, 1907). 



These four forms, whose distinctness and permanence I have demon- 

 strated beyond a possible question, have been studied in their hybrid com- 

 binations, and it is the cultures of these and their hybrids with which the 

 present paper will mainly deal, though some features of other cultures will 

 be discussed. 



It is doubtful whether any bit of technique that has been recently 

 added to the tools with which the biologist may operate in unlocking the 

 mysteries of protoplasmic organization is of so far-reaching- importance 

 as the process of hybridization. I say has been recently added because, 

 although the art of producing hybrids is very old among the breeders of 

 animals, and nearly 200 years old among gardeners, little of scientific value 

 could be secured by means of hybridization until some of the fundamental 

 laws involved in the process were recognized, and it is only within the last 

 eight years that biologists have gained sufficient insight into the behavior 

 of hybrids to give interpretations of the results of hybridization any value 

 as indications of protoplasmic structure and behavior. 



Two general types of hybrids are readily recognizable, namely, the con- 

 stant and the splitting. The former may or may not give indication of the 

 characteristics of their parents, being usually, but not always, intermediate 

 between the parents in most of their characters. The splitting hybrid 

 always indicates by its offspring what were the characteristics of its im- 

 mediate ancestors. It is the latter type of hybrid which is of the greatest 

 usefulness in giving- insight into the structure of the germ-plasm. Hybrid- 

 ization in such cases does not only serve to unite in the same individual all 

 of the characteristics of both parents, but as successive generations are 

 followed, it results in a complete analysis of all the points of difference 

 existing in the two biotypes between which the cross was made. 



The conduct of such analysis by hybridization is particularly simple in 

 the case of plants which, like Bursa, readily self-fertilize, because once the 

 first-generation hybrid is secured the process of analysis goes on genera- 

 tion after generation, until all the allelomorphic differences of the parents 

 are made manifest by being separated in a pure state in different individuals. 



In some respects even more satisfactory evidence of the elementary char- 

 acter of two forms is to be derived from their behavior when crossed than 

 from their conduct in straight pedigrees. If, for instance, two forms sup- 

 posed to be elementary to each other should be, instead, merely extreme 

 fluctuants of a single biotype, their cross-bred progeny would show a fluc- 

 tuating series, including" perhaps the two parental extremes, but could hardly 



