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77/ A' . I ME RICA N XA TURALIST [Vol. XLIV 



1. The differences may be due merely to "coincidence" 

 or "accident." The odds against such an occurrence 

 have been shown to be high. Indeed the cumulative im- 

 probability that all of these differences have been acci- 

 dental is enormous. 



L\ They might have resulted from a slight though 

 constant biasing of the measurements in favor of that 

 result which was calculated to give the greatest personal 

 satisfaction. This possibility, which has already been 

 considered, has been excluded in the case of the second 

 series of measurements. 



3. Granting their genuineness, the differences may be 

 due, not to any specific influence (hereditary or other- 

 wise) which has affected the tail, foot or ear directly, but 

 to some general constitutional difference in the offspring 

 of the two sets of parents. In other words, these differ- 

 ences in the length of the peripheral parts may be corre- 

 lated with some constitutional difference of a very gen- 

 eral sort. In this connection, it must be admitted that 

 the offspring of the warm-room mice showed a very much 

 higher mortality (40 per cent, between the first and sec- 

 ond measurements) than those of the cold-room mice 

 (20 per cent.). The former were likewise somewhat 

 larger, on the average, when measured at the age of three 

 months. Thus there did exist some sort of a constitu- 

 tional difference. The possibility here considered can 

 not, therefore, be set aside. On the other hand, there is 

 absolutely no evidence in its favor. 



4. An explanation closely similar to the last would be 

 that the general stage of development in one lot of mice 

 had been accelerated or retarded as compared with that 

 of the other. We know that the ears and feet of young 

 mice are relatively much larger than those of older ones. 

 It might be contended, therefore, that the warm-room 

 descendants were in a relatively more juvenile condition, 

 despite the fact that they were, on the average, no smaller 

 (larger, indeed, at the time of the later measurements). 



