36 THE QUANTITATIVE METHOD IN BIOLOGY 



investigation of heredity ; Mendelism teaches us more than 

 this. 



§ 34.— MENDELISM {continued). THE NOTION OF 

 SPECIES. — From MENDEL'S experiments and the numerous 

 experiments carried out by his successors we are allowed to 

 draw the conclusion that the groups of specimens which we call 

 species (or subspecies) are distinctly different from one another. 

 The belief that intermediates (transitions) between the species 

 exist is an unavoidable consequence of the proposition that sub- 

 species, species, etc., arise through the transformation of masses 

 of individuals by the selective accumulation of minute differ- 

 ences (impalpable changes).^ As often as intermediates (trans- 

 itions) are proved not to exist, the above proposition about 

 the origin of species ^ cannot be accepted. 



In fact, there is ambiguity in the use of the term intermediate. 

 A distinction ought to be made between intermediate with regard 

 to the properties and intermediate with regard to the possibilities. 

 (See § 21.) 



Intermediate forms (individuals) in respect to one or several 

 observable properties are very common and this has brought 

 about a misconception. Those intermediate forms are, in the 

 state of nature (hybridization being excluded), a result of plas- 

 ticity (§ 8a). They appear and disappear again and again as 

 often as the conditions of existence are altered. (See, on the 

 fugitiveness of observable properties, the examples mentioned 

 in § 12.) 



In other words, two species, observed under certain condi- 

 tions of existence, may be seemingly connected by specimens 

 with intermediate properties. Between both, however, a 

 GAP exists with regard to their possibilities (which depend 

 on qualitative differences in the composition of their living 

 mixture). Although the gap may be narrow, it is permanent : 

 there is no continuity. 



This view, drawn from the study of innumerable examples of 

 plasticity, and from the study of systematic natural science, is 

 supported by numerous facts discovered by MENDEL and his 

 successors. The distinction between properties and possibilities 

 becomes evident by the observation of hybrids. 



EXAMPLE : When two species (subspecies) are crossed 

 which differ in one pair of properties D (dominant) and R 

 (recessive) a new living mixture is produced and two possi- 

 bilities exist in the offspring. In the first hybrid generation 



1 "Once for all, that burden so gratuitously undertaken in ignorance of genetic 

 physiology by the evolutionists of the last century may be cast into oblivion." 

 BATESON, loc. cit., p. 289. 



2 Just as any other theory explaining the origin of species by the accumula- 

 tion of impalpable changes. 



