18 



INTRODUCTION. 



5. Instability and Mendelian Inheritance of 

 Hybrids and Mutants. 



Pocke's data show that instability is usually quite 

 marked in hybrids, especially in hybrids that are the 

 offspring of a number of species and of crossed hybrids. 

 As has long been known, there is no characteristic of 

 hybrids that has been found so undesirable to the plant- 

 breeder as the tendency to vary in succeeding generations, 

 especially in the direction of reversion to one or the 

 other parent. The partial or complete absence of fixity 

 following the first generation was merely a matter of 

 speculation until the contributions of Mendel (1865 to 

 1870), which, however, remained practically unnoticed 

 until 1900. Mendel's discoveries and his conceptions of 

 xmit characters and their mode of inheritance have 

 offered in an important but restricted measure explana- 

 tions for the common failure of many plant and animal 

 breeders to anticipate with any degree of certainty sev- 

 eral results that may under certain conditions be ex- 

 pected by crossing and in successive generations of the 

 offspring, especially in the case of certain kinds of 

 parents. Mendel recognized that hybrids, as a rule, are 

 not exactly intermediate between the parent species, and 

 that while with some of the more striking characters in- 

 termediateness is seen, with others one of the parental 

 characters is so preponderant that it is difficult or im- 

 possible to detect the other in the hybrid. He was the 

 first to show that in order to be able to predict with 

 sureness certain characters of the hybrid it is essential to 

 start with pure stock; study each character separately 

 as an individual unit; group the characters in contrast- 

 ing pairs, one of which pair tends to be transmitted 

 entirely or almost unchanged (dominant character), 

 while the other tends to lessened development (recessive 

 character) or to entirely disappear, but to reappear un- 

 changed in their progeny ; look upon each pair as being 

 independent of the others in heritability ; and regard 

 each generation of offspring as a distinct entity, but in 

 association with the characters of preceding and succeed- 

 ing generations. Mendel found that the hybrids in their 

 various macroscopioal characters, singly and collectively, 

 either closely resemble or are almost identical with one 

 or the other parent species, or are intermediate between 

 the parents ; that the hybrid may exhibit greater luxuri- 

 ance of growth; that the hybrid Seeds are often more 

 spotted (the spots even coalescing in patches) than in 

 the parents ; that the dominant character may be paren- 

 tal or hybrid in character and, if the latter, maintain 

 the same behavior in the second generation ; that the hy- 

 brids resulting from reciprocal crosses are formed alike 

 and exhibit no appreciable difference in subsequent de- 

 velopment; and that in the first and succeeding genera- 

 tions bred from seeds of hybrids there appear in the 

 offspring both dominant and recessive characters of con- 

 trasting pairs in definite average or mathematical 

 proportions. The hybrids of varieties were found to 

 exhibit peculiarities like those of species, but with greater 

 variability of form and greater tendency to reversion | 



to the original types. Mendel's statement that the re- 

 sults of reciprocal crossing are identical must be taken 

 as having a very limited application, and then only in 

 a very gross sense. 



The Mendelian doctrine has found a wide though 

 limited application in the explanation of the various 

 phenomena of heredity, and it seems probable that when 

 all or a large number of parental and hybrid characters 

 of given parents and offspring are studied it will be 

 found to be applicable to a fewer number of characters 

 than is generally believed and of little importance in 

 explaining the phenomena of heredity under natural con- 

 ditions. In fact, the Mendelian doctrine deals with 

 inheritance and not with origin of characters and it 

 absolutely fails in so far as the possibility of the origina- 

 tion of new characters is concerned, and hence is useless 

 in accounting for the occurrence of characters in the 

 hybrid excepting by dominance, recession, and redistri- 

 bution of preexistent ancestral characters. Mendel, while 

 recognizing the commonness of intermediateness of 

 parental characters in the hybrid, made no attempt to 

 apply or extend the doctrine to the explanation of blended 

 inheritance. In fact, he recognized that his doctrine 

 was not applicable to characters that blend. In recent 

 years several investigators have suggested a Mendelian 

 interpretation of blended inheritance. Nilsson-Ehle 

 (Lund's Universitets Arsskrift, 1909, v, 2) holds the 

 view that such form of inheritance is really a segregated 

 inheritance due to the association of several independent 

 but similar imits or factors which yield a pseudo or actual 

 blending. 



The general assumption by pro-Mendelianists that 

 unit characters are constant and changeless has been 

 shown by Castle (American Breeder's Magazine, 1913, 

 III, 270; American Faturalist, 1912, xlvi, 352) to be 

 without warrant, and that, to the contrary, unit charac- 

 ters are variable and modifiable. It is well known that 

 a hybrid has characters that may or may not be inter- 

 mediate, and that may even be peculiar to itself, and 

 that it is the sum of such characters thaA gives hybrids 

 the characters of elementary new species, of which an 

 illustration will be found in our histologic and micro- 

 scopic study of Ipomcea sloteri in Part II, Chapter II. 

 Plasticity of characters as regards degree of develop- 

 ment, fixity, and genesis has long been recognized as one 

 of the most essential fundamental properties of living 

 matter. Development of various characters exceeding 

 that of the parents has been frequently observed among 

 both hybrids and mutants. Increased virulence of suc- 

 ceeding generations of bacteria was pointed out by Pas- 

 teur, Chamberland, Eoux, and many others. Loss of 

 characters is of too common an occurrence to demand 

 special notice. Modifiability, genesis of new characters, 

 and heritability of both modified and new characters have 

 been recorded by a number of investigators. 



Massini (Archiv f. Hygiene, 1907, lsi, 250) culti- 

 vated a strain of Bacteria coli mutabile that gave rise 

 through successive partial mutations to colonies that fer- 

 mented lactose and (in the course of successive genera- 



