2/6 F. B. SUMNER. 



not over 17 days old, i. e., when the young animals had just 

 opened their eyes. At 4 and 6 days, the young are, of course, 

 blind and helpless. Indeed, in two cases were further proof 

 needed it will be noted that the first brood contained no males, 

 while in still another, they had not yet been born. 



In cases of the second class (Table II.), the first and second 

 litters were born 63 to 8 1 days apart, i. e., the second conception 

 occurred when the first brood which in every case contained a 

 male was 41 to 59 days old. In these cases, the possibility is 

 by no means excluded that a member of the first litter served as 

 the father of the second. As already stated, there is positive 

 evidence, both in the white mouse and in Peromyscus, that 

 fertilization may occur at an age of six weeks. On the other 

 hand, such precocity is quite exceptional, so that this explanation 

 of the cases in Table II. seems hardly more probable than that 

 by which we must explain those in Table I., viz., fertilization by 

 spermatozoa received from the earlier mate. In the absence of 

 definite evidence, however, it would be idle to speculate regarding 

 the relative probability of these two interpretations, and I shall, 

 accordingly, exclude the cases in Table II. from most of the 

 ensuing discussion. Nevertheless, it is worth pointing out 

 what is not indicated in the table that the first births recorded 

 in cases 3 and 4 of Table I. are the same as the second births 

 recorded in cases 5 and 9 of Table II. If, therefore, these latter 

 are likewise instances of deferred fertilization, we must assume 

 for these two mothers the delivery of three consecutive broods, 

 after separation from their mates. The possibility of such a sur- 

 prising occurrence surely warrants more carefully controlled 

 experiments in the future. 



In the more familiar discussions of superfetation, it appears 

 to be invariably assumed that the later fetuses owe their existence 

 to coition occurring during the first pregnancy. This assumption 

 is made by Aristotle, in the passages referred to. Marshall 

 (1910, p. 159) says: "If ovulation takes place during pregnancy, 

 and if, owing to the occurrence of coition the ova become fer- 

 tilized, the phenomenon of superfoetation may take place that 

 is to say, foetuses of different ages may be present in the same 

 uterus." 



