MODERN BIOLOGY 589 



conception of heredity and will always, therefore, retain its historical 

 significance." 



WiLHELM LuDviG JoHANNSEN was bom in 1857 at Copenhagen, studied 

 there and in Germany, and eventually became a professor, first at the In- 

 stitute of Agriculture and afterwards at the University in his native country. 

 Being a pupil of PfefFer, he first of all went in for experiments in plant physi- 

 ology, making a number of interesting observations in this sphere in connex- 

 ion with the effect of ether on the metabolism and growth of plants. Soon, 

 however, he devoted himself exclusively to experimental heredity-research 

 and has gradually become one of the leading authorities in that field. Origi- 

 nally a supporter of Galton's statistical method, he quickly realized its 

 deficiencies: that by working with mixed m.aterial it reached conclusions 

 utterly at variance with the true facts. Starting from de Vries's insistence 

 upon the necessity of investigating the units of hereditary characters, he 

 began to follow with minute care the phenomena of heredity in generations 

 of plants. He purchased a quantity of beans, weighed them, and then culti- 

 vated the seeds of every bean separately; he thereupon found that within 

 a succession of individuals thus produced — a "pure line," as he called it — 

 there exists a certain type of hereditary units, which remains unchanged 

 throughout; if the plants are starved, both they and their seeds become small; 

 if they are manured, they grow strong, but this has no effect upon the heredi- 

 tary character; whether one sows small or large seeds of the same pure line, 

 one obtains under the same external conditions the same plant-type. There 

 is thus within one and the same pure line a certain hereditary type — a 

 "genotype," as it is called — that is unalterably the same, whether or not 

 the vital conditions alter the external form of the actual individual — that 

 is, the phenomenon-type, or "phenotype," as Johannsen called it. Those 

 characters which form the genotype are thus the only ones that are really 

 hereditary, whereas the phenomenon-type produced by environment has 

 nothing to do with heredity; stunted growth in generations of plants grow- 

 ing on poor soil is an external character, a "false heredity." There is there- 

 fore no possibility of acquired characters' being inherited, nor is there within 

 the pure lines any chance of variation of the kind assumed by Darwinism. 

 Those characters that go to make up the hereditary disposition Johannsen 

 terms hereditary factors, hereditary units, or genetic elements; in a pure 

 line, then, the genetic elements are the same: its individuals are homozygotes 

 ("zygote" denotes the fusion-product of the male and female sexual cells), 

 while the offspring in life-forms that multiply by the pairing of different 

 individuals is a heterozygote, as it represents a fusion of the parents' vari- 

 ous inheritable factors. A heterozygous individual therefore always has a 

 hybrid nature, and special methods are necessary for the elucidation of its 

 qualities. But at this point we come to the subject of hybrid research, which 

 possesses a history of its own. 



