FUNCTIONAL VARIATION 105 



know that as long as the white corpuscles are able to discharge 

 their proper function the resistance is complete. Why do they 

 weary of their work, and what condition is left behind which 

 assures absolute resistance to future invasions ? 



The phenomenon of acclimatization in general represents a 

 condition in which an organism has undergone a permanent 

 change in its vital functions, forced upon it by the exigencies 

 of life. In the future studies, however, it will be seen that the 

 disturbing effect of adverse conditions, if not too severe, may be 

 gradually overcome, and the animal or plant resume its functions, 

 either modified or unmodified ; and it will be seen further that if 

 the changes be gradual, the immunization will extend to a point 

 that would have been fatal at the outset. Thus organisms may 

 be reared in a gradually intensified poisonous solution, or in a 

 liquid whose temperature is slowly raised, and in this way a point 

 may be reached many degrees above the power of normal organ- 

 isms to withstand. The subject cannot be pursued further in 

 this connection, for it is a large one, with many other bearings ; 

 but the student should bear it in mind throughout the study. 



Irregular functioning. An interesting phase of irregular func- 

 tioning is found in the so-called "instinctive acts," more properly 

 reflex actions, which by popular conception are supposed to pro- 

 ceed with unerring accuracy. This assumption is natural in view 

 of the complex nature of many of these acts, all of which have 

 the appearance of being under the control of reason. For exam- 

 ple, note the complicated nature of the process necessary to the 

 successful deposition of the egg of the yucca moth (Pronuba yuc- 

 caselld). We are told 1 that these moths emerge simultaneously 

 with the flowers of the yucca, which open but for a single night 

 and are practically dependent upon this particular moth for ferti- 

 lization. When ready to oviposit, the female gathers a bundle of 

 pollen from one flower, flies with it to another, pierces the tissues 

 of the pistil of the latter, and lays her egg ; after which she 

 ascends to the stigma of the same pistil and " stuffs the fertilizing 

 pollen pellet into its funnel-shaped opening." 



Now this process is necessary not only to the fertilization of 

 the yucca, but also to the grub that hatches from the egg, which 



1 Morgan, Habit and Instinct, pp. 13-15. 



