50 MICHAEL F. GUYER 



So far as the writer has carried his experiments, the indi- 

 cations are that on the whole there are more brown than white birds in 

 the third generation, and this points to the conclusion that in the brouTi 

 birds we may have both intermediate forms like the hybrids of the second 

 generation and forms which have reverted to the brown grandparent, 

 as the white doves have seemingly returned to the white grandparent. 

 . . . . The birds of this generation, then, might mate in such a way 

 that the offspring could exhibit the ancestral white while yet remaining 

 intermediate in other characters. As we shall see in the conclusions 

 from the study of the germ-cells of hybrids, there are certain phenomena 

 in the germ-cells which apparently afford us a definite physical basis 

 for the production of intermediate forms and for returns to pure ancestral 

 species. From this basis there must necessarily be a greater number of 

 intermediate forms in the offspring of hybrids than there are reversions 

 to the respective ancestral species. 



I had interpreted the irregular phenomena occurring in hybrids 

 at the time of synapsis as due to the tendency of the chromatin 

 of each parent to retain its own individuality. And while I had 

 attributed some importance to these irregularities of division in 

 accounting for variations and reversions in the third generation, 

 I did not regard them as the chief factors in such returns, as is 

 evident from the following excerpt (p. 47) : 



In discussing irregular divisions, however, it must not be forgotten 

 that many apparently normal divisions of the spermatocytes also occur 

 in hybrids, and constitute by far the predominant kind of division in 

 hybrids from closely related forms. Unequal distribution of chromatin 

 can not therefore play the most important part in variation or reversion. 

 There seems to be no other interpretation, indeed, than that in the many 

 normal mitoses of the bivalent chromosomes which occur, the chromatin 

 of the father and of the mother is set apart so that the ultimate germ- 

 cells are what might be termed 'pure' cells; that is a given egg or sperm- 

 cell contains exclusively or at least predominantly qualities from one 

 parent. The offspring from fertile hybrids of the same parentage might 

 then be similar to the mixed type of the original hybrid, or revert to one 

 of the grandparent tyj^es, dependent upon the chances of the various 

 cells for union at fertilization. If a spermatozoon and an egg containing 

 characteristics of the same species unite, then the reversion will be to 

 that species; if a sperm-cell containing the characteristics of one species 

 happens to unite with an ovum containing characteristics of the other 

 species, then the offspring will be of the mixed t>pe again. By the 

 law of probability the latter will be the more prevalent occurrence, 

 because there are four combinations possible, and two of the four would 

 result in the production of mixed offspring, while only one combination 

 could result in a return to one of the ancestral species. 



