SUTTON 



dealt with in the early part of this 

 paper. No mention is made in the 

 paper of Mendel's law but in consider- 

 ing the inbred pigeon hybrids from 

 which his material was obtained, the 

 author expresses his familiarity with 

 manifestations of the Mendelian prin- 

 ciple by the statement that "in the 

 third generation there is generally a 

 return to the original colors of the 

 grandparents." In cases which seem to 

 resemble one grandparent in all partic- 

 ulars it is clear that the conception of 

 pure germ-cells may be strictly ap- 

 plied, but the author was familiar with 

 cases of inbred hybrids which plainly 

 show mixtures. These he is inclined to 

 explain in two ways as follows: (1) 

 "Union of two cells representing each 

 of the two original species would yield 

 an offspring of the mixed type." (2) 

 "Besides through the mixing just indi- 

 cated, variability may be due also in 

 some cases to the not infrequent in- 

 equalities in the division of individual 

 chromosomes, through which varying 

 proportions of the chromatin of each 

 species may appear in certain of the 

 mature germ-cells" (p. 20). 



The first of these explanations would 

 accord with the result of Mendelian 

 experiment but for the fact that it is 

 erroneously applied (and without 

 cytological grounds) to all the char- 

 acters or chromosomes instead of to 

 individuals. As for the second passage 

 ■quoted, there can be little doubt that 

 irregular division of chromosomes 

 would be likely to produce marked 

 variation, but as Guyer himself ob- 

 serves, these irregiihwities increase with 

 the degree of infertility. It seems nat- 

 ural to conclude, therefore, that they 

 are not only pathological but perhaps 

 in part the cause of the infertile condi- 

 tion. Furthermore, on the hypothesis 

 of individuality of chromosomes, 

 which Guyer accepts, the loss of a 

 portion of a chromosome by irregular 



41 



division would be permanent and the 

 effect of repetitions of the operation 

 upon the descendants of a single chro- 

 mosome group (which he regards as 

 transmitted as a whole) would be so 

 marked a depletion of chromatic sub- 

 stance as must lead soon to malfunc- 

 tion and ultimately to sterilit)\ 



As already noted (p. 216) the first 

 of these two explanations of the causes 

 of variation would allow only four 

 possible combinations of chromosomes 

 in the offspring of a single pair. But 

 we know that except in the case of 

 identical twins, duplicates practically 

 never appear in the offspring of a pair 

 however numerous the progeny. 

 Therefore, whatever the number of 

 the offspring, the variations of all ex- 

 cept the few provided for by the four 

 normal chromosome combinations 

 must be accounted for by obviously 

 pathological division processes, which 

 tend strongly in the direction of steril- 

 ity. But in the report of Bateson and 

 Saunders to the Evolution Committee 

 we find the statement: "We know no 

 Mendelian case in which fertility is 

 impaired" (p. 148). When we reflect 

 that the vast majority of cases studied 

 by these observers were Mendelian and 

 connect this piece of evidence with 

 the testimony of Cannon ^^ that the 

 maturation processes of variable cot- 

 ton-hybrids are either normal or so 

 distinctly abnormal as to entail sterilit\' 

 and with Guyer's own admission that 

 the abnormalities in mitosis increase 

 with the degree of sterility, the bal- 

 ance is strons^ly against the efficacy of 

 pathological mitoses as factors in nor- 

 mal hybrid variation. 



I take pleasure in acknowledging my 

 indebtedness to Professor E. B. Wilson 

 for invaluable counsel in the presenta- 

 tion of a subject offering many diffi- 

 culties. 



28 Cannon, W. A., loc. cit. 



