400 CLARENCE HAMILTON KENNEDY. 



small number of units (either cells or combinations of cells) 

 that comprise it. Such simple nervous systems obviously cannot 

 produce the extended series of finely graduated reactions that 

 are possible to a more complicated type of system. On the other 

 hand the mechanism is already present for the reversal of any 

 particular reaction. So a complete reversal of a generic reaction 

 Avhich puts the species entirely outside of the normal generic 

 habitat, is more likely than the slight modification necessary 

 to put it into a near but only slightly different habitat in which 

 it would be held by only a slight gradation of the general reaction. 



At first thought, if it is easier to have a reversed reaction in 

 the insect nervous system than to have a graduated reaction, one 

 might think that the genus would fly apart as to any unity of 

 environment, so that each species would have a habitat strikingly 

 different from that of each other species, that there would be 

 no such thing as a generic habitat. Observation shows that 

 this is not so. Species are superimposed and habitats overlap 

 in a most confusing manner and in any large genus there is 

 usually an easily recognized generic type of habitat. So far 

 not enough experimental work on behavior has been carried out 

 to determine if the species of any one genus of insects are dis- 

 tributed by graduated reactions to one type of stimulus. How- 

 ever an analysis of some of the factors of the problem appear to 

 indicate that such a distribution, for instance as the species of 

 sEshna, show in a series of temperature gradations, may not be 

 due to a slight difference in reaction to a specific stimulus, but 

 may be merely an apparent series each species of which is held 

 in its particular zone by some different positive or negative 

 reaction to any one of a variety of stimuli. . 



An insect with incomplete metamorphosis passes through two 

 stages, nymph and adult, in each of which it may pass through 

 several physiological stages of development. The nymph has at 

 least two, its feeding stage and its quiescent preemergence stage. 

 The imago has at least five stages, the teneral, the sexually 

 mature stage, which can be divided into periods of hunger, 

 periods of sexual lust, periods of egg-laying, and finally the stage 

 of senility. The last need not be considered in a problem of 

 species distribution as it is beyond and outside the genetic cycle. 

 In each of these stages several stimuli control the individual, 



