342 



The Journal of Heredity 



problems of heredity before the mind. 

 It is easy to admit in general terms 

 that there must be something in the 

 germ-cells to correspond to what we 

 call characters in adult organisms, but 

 to make this general admission is very 

 different from forming a concrete idea 

 of the hypothetical mechanism of hered- 

 ity. We may be as sure as ever that 

 organisms are machines, but reiteration 

 of confidence in the general mechanical 

 ])ostulate does not give us increased 

 insight into the machinery. The fact is 

 that we are still without any definite 

 indications of the nature of the un- 

 formed, rudimentary characters that 

 are supposed to exist in the germ-cells. 

 The many names that have been given 

 to those hyi)othetical entities, such as 

 pangen, determinant, id, character- 

 unit, gen, etc., are merely symbols for 

 the unknown quantities of biology. 

 Their only use is for the statement of 

 biological problems. Instead of writing 

 OUT X with a single character as in 

 algebra we use the symbol gen, with 

 three letters. A gen is the unknown 

 something that is assumed to re]jresent 

 a latent or unexpressed character; that 

 is, a character as it is supposed to exist 

 in the protoi:)lasm, before it comes into 

 expression during the dcveloi)mcnt of 

 the organism. 



MENDELISM A MONOGENIC THEORY. 



The so-called Mendelian theor}- of 

 heredity is in reality a theory of the 

 formation of germ-cells. To the as- 

 sumjition that characters are transmitted 

 as separate particles or discrete entities 

 of some sort, is added the idea that 

 each germ-cell receives and transmits 

 only a single set of these character- 

 bearing "units." 



The cells that build up the bodies of 

 plants and animals, the so-called "so- 

 matic tissues," are supposed to contain 

 two separate sets of character-units, or 

 gens, derived from the two parental 

 germ-cells that united in conjugation 

 and initiated the development of the 

 new organism. But with each return 

 to the formation of germ-cells the two 

 sets of gens are sujjjjosed to lie broken 

 uj) and redistributed in single sets. If 

 the two series of gens derived from the 



jjarent germ-cells are alike, all of the 

 new germ-cells have the same series of 

 gens. But if all of the gens of the two 

 parental series were not alike, the new 

 germ-cells receive one or the other of 

 the unlike gens, but not both. Thus 

 the germ-cells of hybrids between dif- 

 ferent varieties or species are supposed 

 to remain "pure" with respect to the 

 contrasted characters of the parental 

 stocks. 



On account of this assumption of 

 "pure germ-cells" Mendelism may be 

 described as a monogenic or single-gen 

 theory of transmission. Only one gen 

 of the kinds required to produce the 

 various features of an adult plant or 

 animal is supposed to be represented 

 in the same germ-cell. The failure of a 

 parental character to reappear in some 

 of the members of the second and later 

 generations of offspring is explained by 

 assuming that the character was trans- 

 mitted to only half of the germ-cells. 

 The theory does not provide for the 

 transmission of additional gens to 

 represent characters that may not be 

 brought into expression, or that may 

 give rise to unexpected variations in 

 later generations. To exjjlain such 

 variations Mendelian writers resort to 

 the theory of De Vries, that new char- 

 acters and new species arise by muta- 

 tions. If a sudden change of characters 

 apjjcars in a member of an otherwise uni- 

 form, "pure bred" stock, it is assumed 

 that a new character has been fomied, 

 and that such changes in the char- 

 acters of unifonn groujjs are examples 

 of normal e\'olutionar\' jjrogress. 



ORIGIN OF NEW GENS. 



The effect of these theories is to 

 return to the ])re-Darwinian doctrine 

 of special creation, except that the 

 si)ecies are not sui)posed to be made 

 altogether de novo, but by implanting 

 the gens of new characters in members 

 of old sjjccies. The change of any 

 single character is supposed to establish 

 a new "elementary s])ecies" or "bio- 

 type." How the new gens are made 

 and sul)stituted for the old ones is not 

 ex|)lained, but the mathematical sim- 

 l)licity of the monogenic hypothesis and 

 the idea that the chromosomes or other 



