OF PERIODIC PHENOMENA. 345 



To which may be added 



Time of occurrence of any accidental disease or blight affecting 

 any particular crop. 



(8.) Such are some of the most important periodic 

 phenomena in natural history, which the attentive 

 observer will do well to record. It will be obvious, 

 on inspecting the above table, that there are a few of 

 these, which, in respect of relative date, will be the 

 same or nearly the same every year, and can scarcely 

 be called so much dependent upon season, as upon 

 the date of occurrence of some anterior phenome- 

 non with which they are connected, this last alone 

 being influenced by the cause just alluded to. Thus 

 the exact time of a bird's commencing to lay may 

 vary with the season ; but, this being determined, 

 the periods of incubation, and of the young being 

 hatched and fledged, probably follow regularly in all 

 cases after a given interval. Parturition in like man- 

 ner in mammals is determined by the time of 

 coupling. Also, occasionally, we may have a varia- 

 tion of date, usually dependent on season, but in 

 certain instances varying from other causes. Thus, 

 cock birds which have lost their mate will some- 

 times protract their song considerably beyond the 

 usual time of ceasing. But, in general, the great 

 bulk of such periodic phenomena are directly con- 

 nected with season and climate, and have a range 

 of variation according as these last vary. And 

 this will be particularly the case with plants, from 

 their being attached to the soil. With these there 

 is scarcely one epoch, throughout their several suc- 



Q 5 



