106 VARIATION 



otherwise would be left without food. There is, therefore, a 

 particular sequence in this complicated performance that must 

 be observed or failure results ; and failure is fatal to the exist- 

 ence of both species. 



Moreover, this act is performed but once in the lifetime of 

 the moth, who has no knowledge of the acts of her predecessors, 

 and is therefore not proceeding from simulation ; nor has she 

 opportunity to learn the fate of her offspring and profit by 

 the experience. 



Now the truth is that, unerring as is this performance, a 

 good many ovules escape, from failure of the egg to hatch or 

 from other causes, and thus the yucca is able to mature some 

 seed. That complicated processes of this kind are not always 

 carried out in proper sequence and full detail is shown by 

 careful study of different individuals, as pointed out especially 

 by Professor C. S. Crandall in his studies of the apple and plum 

 curculio. 1 



Careful study of these complicated acts and of the body 

 functions in general must convince the student not only of their 

 nice adjustment but (what is of equal consequence) of their 

 exceeding variability and irregularity within limits that certainly 

 are by no means narrow. 



Cases of "double personality," in which the individual 

 behaves for a time as another and distinctly different person, 

 are too well known to require more than a passing notice. 

 These are instances in which an entirely new set of functions 

 is brought into play, distinct from the normal, yet working- 

 together to the accomplishment of definite ends. 



But a few of the many modifications of normal functions 

 have been mentioned. This is not the place to exhaust the 

 subject. Only enough has been given to show the student that 

 even the highly specialized functions are subject to the laws of 

 variation. The matter will be more completely covered under 

 " Causes of Variation." 



1 Bulletin No. 98, Agricultural Experiment Station, University of Illinois, 

 pp. 500-502. The account of the different ways in which three different females 

 performed their work is given in full under "Transmission of Modifications," 

 section on " Habit and Instinct." 



