May, 1923] 
SHULL — SPECIES CONCEPT 
227 
pastoris that Solms-Laubach, to whom it |vas sent for identification, was 
inclined to place it in the genus Camelina, because of its rounded, uninflated 
capsules. Only when he observed a mutation {Bursa Solmsiana Shull ined.) 
arising from it, which exhibited a partial reversion to the bursa-pastoris 
type of capsule, did he recognize the identity of the new form as a species 
of Bursa. 
I have made a Very large number of hybridizations between B. Heegeri 
and different biotypes of B. bursa-pastoris , and these two species have bred 
together with perfect readiness and with full fertility, so long as the biotypes 
of bursa-pastoris used were of European or east-American origin. The 
segregation showed that a simple Mendelian difference exists between these 
two species, though this difference is quite generally duplicated through 
the presence of the corresponding factor or gene in two different chromosome 
pairs. It is thus seen that genotypically the species-character which 
differentiates B. Heegeri from B. bursa-pastoris is on an exact par with the 
character of leaf-lobing which differentiates the several rosette types of 
Bursa into those which have the sinuses extending to the midrib ( heteris 
and rhomboidea) and those which have the sinuses less deep {tenuis and 
simplex ), which differentials, as we have already seen, can not be used as 
satisfactory characters in taxonomy because they are too easily suppressed 
by environmental factors. It is obvious, therefore, that the capsule form 
is here a satisfactory specific character because it is unmodified by variations 
in the environment, and because it is therefore always available for use in 
determining the identity of any individual, while the genotypically equal 
differences in leaf-lobing are not suitable specific characters because they 
lack this quality of being invariably expressed when the genotypic constel¬ 
lation requisite for their expression is present. 
Since there is no natural hiatus in the range of visibility of different 
phenotypic characters possessed by different individuals or groups of plants 
and animals, nor in the degree of their persistence under the normal varia¬ 
tions of the environment, I believe that my statement, made above, is 
substantiated, that species are only quasi-natural entities and that they are 
made so by the lack of agreement between external appearance and internal 
constitution and by the low visibility of many hereditary characteristics. 
Natural groups there certainly are, but these are the biotypes of the geneti¬ 
cist, not the species of the taxonomist. Only here and there is there a 
coincidence between biotype and species. 
Since the usefulness of the species concept rests upon the exchange 
value of specific names in scientific and intellectual intercourse, it must be 
borne in mind that an undue increase in the number of species has the same 
effect on the exchange value of the species in the intellectual markets of 
the world that analogous inflation of financial currency has upon the value 
of any monetary unit involved in such inflation. Compare, for example, 
the usefulness of Crataegus species today with the decline in value of the 
