56 I'EklOlJiCITV IX ORtlANIC LIFE. 



This plant's appearance is a good example of the abundant 

 period. It will be remembered how alarmed those who had charge 

 of the canals were, and how they were at their wits' end, to find 

 out some means for reducing the mischief ere the canals were 

 rendered quite impassable. Our native water-plants were choked 

 and destroyed, and it seemed as if the new-comer was going to 

 appropriate all our waters, to the destruction of everything besides ; 

 but the period of prosperity is over ; the plant is gradually becoming 

 less of a pest ; our British plants are again able to assert and 

 maintain their proper place ; our waters are becoming less clogged, 

 and let us hope that we are approaching the period of scarcity of 

 this species, which we could so well spare. How can the extreme 

 abundance of this species be accounted for, and how can its gradual 

 diminution be explained? The waters are as before, their 

 ingredients are the same, and the surroundings have not altered, but 

 still Anacharis is apparently dying out. This may be a case of 

 periodicity, and if so, this water pest will some day again be growing 

 in the same rampant condition. 



Who is there, dwelling in the country, of observant habits, that 

 has not noticed the abundance at uncertain intervals of the common 

 thistle. In those seasons it may be seen everywhere, not only in the 

 badly-cultivated and neglected fields, but also in pastures which 

 have been for years fed down by cattle only, and where the soil is 

 undisturbed also, and no manure put on. This condition may exist, 

 and does do so frequently, over large districts. After a time it may be 

 noticed that the thistles are less abundant, and then for years only 

 the normal quantity may be seen. 



It must be understood I am not referring to such arable lands as 

 may be neglected and out of cultivation, for here weeds of all kinds 

 are sure to abound. Neither do I believe that this periodic 

 abundance is caused by the seeds that are blown from neglected 

 spots, because, some years since, when every part of Essex was 

 well and carefully cultivated, the extreme prevalence of this weed was 

 noticeable for a few seasons in succession. This periodic abundance 

 occurs in many plants, but the examples given may be sufficient to 

 illustrate my meaning. 



All members of the Essex Field Club collecting any of the orders 

 of insects, must have noticed that for several successive years 

 certain species are rare, and that after an interval, longer or shorter, 

 they begin to find the individuals of these species more commonly. 



