PERIODICITY IN ORGANIC LIFE. 6 1 



considered as due to the agency of man, to what are we to attribute 

 the abundance of this bird during the last few years ? I am not 

 aware that man has destroyed any of its enemies, nor am I aware 

 that he has cultivated any fresh crop which can have been utilized 

 by it for food ; nor do I know that meteorological conditions have 

 been peculiarly favourable. The only way of explaining it is by 

 stating the fact that this species is passing through its period of 

 abundance. 



The Goldfinch is an instance of a species passing through, one 

 must hope, a period of scarcity only and not of approaching 

 extinction. The agency of man may possibly have some effect in 

 producing a portion of this rarity, but he certainly is not the sole 

 cause. 



Amongst the migratory birds we often see a great abundance at 

 times, sometimes for two or three or more years in succession. 

 There may have been in the district from w^hich they came abund- 

 ance of food or other favourable causes, which may account for 

 their increase, and therefore for the large numbers of our annual 

 visitors. Sometimes a greater degree of cold would appear to be 

 the explanation ; but we must remember there are cold seasons 

 when the flocks are few, so that cold weather cannot be the only 

 factor. I have no doubt that if we could get a correct estimate for 

 every part of the globe visited by these northern breeders, we should 

 find the same inexplicable periodicity of abundance and the reverse 

 obtaining as the rule with these creatures as with all the classes we 

 have been considering ; and that the numbers we see have nothing 

 to do with either heat, cold, or abundant food. 



The next great division of nature, Mammalia, will furnish us with 

 many examples of the law of periodicity, and we shall in these be 

 able to follow the various apparent means by which the abundance 

 or scarcity is occasioned ; but we cannot tell even then how these 

 means were brought about. This we can do from the more 

 stationary habits of the creatures, and from the large number under 

 the control of man. 



Plenty, as regards food and favourable weather, will be found 

 very active agents in the welfare of, and abundance of, this class ; 

 but meteorological agents, as far as we can see, have but little 

 influence. This I will illustrate by reference to a domestic animal 

 first. The expression " a good fall of lambs " is well known, and 

 it is a matter of observation that there are seasons when a very 



