DEFINITE DIFFERENCES 85 
characteristics as those which were exhibited by the 
two original parental strains. The only difficulty in the 
way of invariably distinguishing the two original kinds, 
after their segregation in the offspring of the cross, lies 
in the fact that the smallest seeds of the large type 
may be smaller than the largest seeds of the smaller 
strain, and this is a difficulty which applies equally 
to the original strains before crossing, as well as to the 
case of the evening primrose fruits just mentioned. 
Now, it is clear that if we mixed together the seeds 
of several different races of peas in the proper pro- 
portions, the result might lead to a normal distribution 
of the kind presently to be described. The several 
races, however, would none the less be perfectly distinct, 
even though we could not separate the individual 
seeds belonging to each by any direct method.* Sucha 
mixture of races would constitute a decided pitfall for 
the unwary statistician, and it is well to remember 
that, after even the most elaborate mathematical 
analysis, the final result cannot be clothed with any 
greater amount of certainty than the facts from which 
the calculation set out. Those who have made a large 
expenditure of intellectual effort in such processes have, 
unfortunately; a natural tendency to overlook this 
elementary fact. 
Prior to the application of statistical methods to a 
particular case of normal variation a number of pre- 
liminary processes have to be gone through. 
* It would generally be possible to decide which strain a 
particular seed belonged to by sowing it and observing the 
variation of its offspring. 
