348 THE ZOOLOGIST. 



less swarms. They have a host of enemies, and Buzzards, Owls, 

 Ravens, and other predaceous birds thin their ranks by hundreds ; 

 while among four-footed foes, Polecats and Stoats follow the 

 track of the advancing legions, and kill them where and when 

 they can. The Polecat and Stoat are, moreover, able to follow 

 the Hamster into the recesses of their burrows, where they 

 probably destroy them by hundreds.* 



Innumerable illustrations might be chosen from the life- 

 histories of insects. Prof. Miall observes : — " Winter, of course, 

 brings many hardships upon aquatic insects, as the great reduc- 

 tion in their number proves. The enormous number of eggs 

 laid by so many of them is doubtless connected with the heavy 

 risks to which they are exposed during half the year." f Of one 

 of the May-flies (Polymitarcys virgo) Reaumur states: — "The 

 short life of the winged female compels her to deposit her 700 or 

 800 eggs at once, without much discrimination of likely and 

 unlikely places." Of the Pine Sawfly (Lophyrus pini), whose 

 larvae are frequently found in such numbers in pine-woods, it has 

 been said : — " When young, and also just before turning into 

 pupae, the grubs are very susceptible to sudden cold or heavy 

 rain, which kill off thousands. In addition to these destructive 

 agencies, nearly forty different kinds of parasites infest the grubs, 

 while mice devour numbers of the pupae."! Among the Thread- 

 worms (Nemathelminthes) parasitic Nematodes produce enormous 

 numbers of eggs. Van Beneden states that 60,000,000 have 

 been computed in a single Nematode, and this multiplication of 

 ova is absolutely necessary, for the chance of the embryo reaching 

 the right host, in which alone it can develop, is always a 

 small one. § 



This excessive fecundity in some animal life finds its parallel 

 in plants. Thus it has been computed that a plant of Sisymbrium 

 sophia yields 730,000, one of Nicotiana tabacum 360,000, one of 

 Erigeron canadense 120,000, and one of Capsella bursa- pastor is 

 64,000 seeds yearly. || Probably in this case, and in a state of 



* Lydekker, ' Eoy. Nat. Hist.' vol. iii. p. 126. 



f ' Nat. Hist. Aquatic Insects,' p. 18. 



J F. O. Pickard-Cambridge, ' Koy. Nat. Hist.' vol. vi. p. 17. 



§ Of. A. E. Shipley, ' Cambridge Nat. Hist.' vol. ii. p. 162. 



|) Kerner and Oliver, ' Nat. Hist. Plants,' vol. ii. p. 878. 



