360 



PSYCHE. 



[March 1896. 



the facts of aphid hibeniation as we 

 find them to-day, we will briefly review 

 the methods outlined in the previous 

 pages and apply to each these principles. 



For our present purpose Professor 

 Lloyd Morgan's term ' natuial elimina- 

 tion'* is more lucid than Dnrwin's 

 'Natural Selection.' Reduced to its 

 simplest form this theory rests upon the 

 fact that " in every generation of every 

 species a great manv more individuals 

 are liorn than can possibly survive ; so 

 that there is a perpetual battle for life 

 going on among all the constituent in- 

 dividuals of any given generation. Now 

 in this struggle for existence, which in- 

 dividuals will be viccorious and live.^ 

 Assuredly those which are best fitted to 

 live : the weakest and least fitted to live 

 will succumb and die, wliile the strong- 

 est and best fitted to live will be trium- 

 phant and survive. "t 



Among the lower animals it is often 

 not so much a struggle between the in- 

 dividuals of a species, as it is with other 

 species and the natural conditions of 

 existence ; not so much a matter of 

 what Morgan calls selection proper, — 

 involving the element of individual or 

 special choice, — as it is a matter of 

 natural elimination. " And the factors 

 of elimination are three: first, elimina- 

 tion through the action of surrounding 

 physical or climatic conditions, under 

 \\ Inch head we may take such forms of 

 disease as are not due to living agencs ; 

 elimination by enemies, including para- 



* Animal Life and Intelligence, p. 80, 



tRomanes, Scientific Evidences of Organic Evolution, p. 3, 



sites ai.d zymotic diseases, and thirdly, 

 elimination b}' competition." * 



In applying liiese factors to explain 

 the hibernation of aphides we must bear 

 in mind the prodigious powers of mul- 

 tiplication possessed by these insects, — 

 because of which tiie autumn progenv 

 of a single stem-mother may amount to 

 millions of individuals. We must also 

 remember that on account of the crowd- 

 ing caused by this rapid rate of multi- 

 plication, it must often happen that the 

 ovipaious females are compelled to de- 

 posit their eggs in all sorts of situations 

 upon the food-plant; and that to-da\ , 

 even when no crowding occurs, the 

 oviparous females often exiiibit a con- 

 siderable diversity in habits of ovi- 

 position. 



Tiie eliminatitig agencies with whicli 

 most aphides in their hilsernating con- 

 dition have to contend apjjear to be 

 chiefly confined to the action of cli- 

 matic conditions and natural enemies. 

 A large proportion of the eggs depos- 

 ited upon smooth bark without special 

 protection must be blown ofl'bv winds, 

 or washed away by rain or melting 

 snow and ice. Species which like 

 the Oak Callipterus and the White 

 Pine Lachnus live upon trees the 

 leaves of which remain upon the 

 branches until the following spring 

 have a decided advantage in oviposi- 

 tion because their eggs are less exposed 

 to dangers of this kind than those 

 which are simply consigned to the 

 bark. This is particulaily true of 



* Morgan, 1. c, p. 80. 



