OF PERIODIC PHENOMENA. 339 



the isothermal* lines, or whether in certain cases 

 with the isotheral f or isocheimal ? J Again : as we 

 may conceive lines upon which a given plant comes 

 into flower at the same time, we might have other 

 lines upon which its flowering is retarded or ad- 

 vanced ten, twenty, or more days ; would these 

 lines be equidistant from each other ? Also, would 

 the lines of synchronic leafing, flowering, and fruc- 

 tification, for the same plant, be generally coinci- 

 dent ? A number of such questions suggest them- 

 selves to the thoughtful observer. And there is 

 no reason why we might not extend the same 

 mode of illustration to periodic phenomena in the 

 animal world. All those points in their history 

 which are dependent upon time and place ; such 

 as their periods of migration and winter sleep, or 

 of nidification and breeding; or, in the case of 

 the insect world, their periods of metamorphosis ; 

 as well as accidental occurrences, coincident with 

 particular seasons, such as the prevalence of cer- 

 tain epidemic diseases, or the unusual abundance of 

 any particular species ; these questions might re- 

 ceive great light from a simultaneity of observa- 

 tions, made with precision in many different loca- 

 lities, and indicated in this manner. We may al- 

 lude to synchronic lines in respect of the first appear- 

 ance of birds of passage, indicating their gradual 



* Places having the same mean temperature, 

 t Places having the same summer temperature, 

 $ Places having the same winter temperature 

 Quetelet. 



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