Periodicity. 121 



locality they most inhabit, as in the case of quails, rabbits, deer, &c. , 

 as well as many insects. But it -is still more remarkable that many 

 creatures imitate not the color merely but the form of others having bet- 

 ter modes of defense. Real wasps are quite conspicuous, but have a 

 weapon in their tail which is a good defense. Species of the Sesia 

 Aegeriidea have no sting, but the} 7 resemble small wasps so closely that 

 everybody is afraid of them. The Butterfly Heliconia is avoided on 

 account of its bad smell. Three other species which do not enjoy the 

 ownership of a bad odor, viz., the Leptalis, Erycina and Ithonia are 

 very like it in appearance, a circumstance which causes them to be 

 avoided by mistake. ( Semper. ) Such cases of imitation are extremely 

 numerous among insects. 



There are many cases in which insects, such as locusts and grasshop- 

 pers, so closely resemble in color and form the green leaves of the trees 

 they inhabit as generally to escape notice. There are also a good many 

 cases of protective resemblances among the vertebrates, where harmless 

 snakes look like poisonous ones, and certain harmless birds look like 

 birds of prey. ' No doubt there are cases of resemblance in animals 

 arising from causes no way related or dependent on each other, and 

 which may be called accidental resemblances, but it is evident that 

 "protective resemblances" however small will sometimes preserve their 

 possessors, and this constitutes a mode of selection. 



CHAPTER XVIII. 



PERIODICITY. 



The only distinct phenomenon attending vitality which seems to sepa- 

 rate it from other forms of motion and chemism, is its power of going 

 on, of continuing its essential qualities under varying forms an egg, 

 enable, child, man, egg thus in four terms completing a cycle of 

 forms each of which in disappearing gives rise to its successor. This 

 uniformity of recurrence is the only thing about it that is characteristic. 

 Growth is not exclusively characteristic of live organisms. Crystalliza- 

 tions, dendritic formations, chemical unions, electrolysis all chemical 

 reactions are examples of growth, which is the quasi spontaneous aggre- 

 gation of matter under its varying affinities and their varying degrees of 

 power. 



But here is a periodicity a going on in one form for a time which 

 is more or less definite, then a change and another period of definite 

 duration, &c. Periodicity goes with everything organic, and is evi- 

 dentl}" originally derived from and dependent on the general periodicity 

 of the great molar movements of the general universe. The growth is 



