BIOTYPES AND HYBRIDS. 5 



iments must be learned by experience, for, as said before, mere inspection 

 of an individual or of a single generation can not distinguish between im- 

 portant and unimportant variations until the pedigrees themselves furnish 

 solutions to the question. For this reason there must be in the beginning 

 a much more minute analysis of the material than is later found necessary 

 or desirable. Most of the cultures which I have made thus far have been 

 directed toward determining what variations are of the fluctuating kind and 

 what are fully transmissible to the offspring. 



In one group I have now had large numbers of plants of the fourth 

 pedigreed generation and a few of the fifth under observation, and in a 

 number of other cases the third generation has been extensively grown. I 

 find that while certain variations which were selected disappear in the first 

 or second generation, others remain constant, easily recognized differen- 

 tiating marks which, except in one form, show no transgression of the char- 

 acteristic features of any other form studied. These forms are, therefore, 

 distinct elementary species, or biotypes, each characterized by certain con- 

 stant features and each with its own normal range of fluctuating variability. 



The systematist has not yet decided what treatment to give to elementary 

 species, and any nomenclatorial scheme must be regarded, therefore, as 

 purely tentative ; but utility can not wait for concerted action on the part 

 of taxonomists in devising a suitable systematic designation of elementary 

 species, and I have therefore for the sake of convenience assigned to these 

 elementary forms of Bursa simple names which can be attached to the 

 accepted specific name to form a trinomial. I was at first inclined to use 

 binomial names which would leave the Linnean specific name, bursa- 

 pastoris, as the valid name of the aggregation of elementary forms having 

 the same general habit and the triangular or obcordate capsules. It might, 

 then, be looked upon as a superspecific name which would remain just as 

 useful in the everyday conversation and experience of the botanist as when 

 the aggregation for which it stands was believed to be a unit. 



The fact that corresponding series of elementary species or biotypes may 

 occur in different related species, as will be shown later in the discussion 

 of the heegeri hybrids, makes the trinomial much to be preferred, for cor- 

 responding forms may then be given the same name without confusion. 

 Thus Bursa bursa-pastoris heteris and Bursa heegeri heteris may be used to 

 denote two forms which are alike in rosette-characters but different in the 

 capsule-character, the latter character being accepted as of specific value. 



That Bursa bursa-pastoris is a composite species was first given public 

 recognition by Lotsy (1906) about a year after my cultures were begun, 

 and his statement did not come to my notice until after I had presented 

 my first account before the American Association for the Advancement of 

 Science at its New York meeting, December, 1906. His statement is very 

 brief, and the chief interest of his account consists in the photographs 



