A REVIEW WITH HYPOTHETICAL CONCLUSIONS. 107 



parasites (particularly micro-parasites), and some predatory enemies fall within 

 this category. Many parasites and most predatory enemies do not. because 

 their efficiency is apt to be governed by extraneous influences more than by 

 the relative abundance or scarcity of the insect upon which they prey. For 

 example, a parasitic insect having two generations annually on two distinct host?, 

 is equally dependent on both. It cannot increase to keep pace with any unusual 

 increase of one host unless the alternate host also increases proportionately. 

 And to exert true " facultative " control it must be enabled to increase a little 

 faster than its host, in order quickly to check what might otherwise be disastrous 

 and self-destructive increase of the latter. 



Thus, in the final analysis, parasites and predators exerting " facultative " 

 control are beneficial rather than detrimental to their hosts ; at the same time, 

 since they must be supported by the sacrifice of a certain — sometimes large — 

 percentage of individual lives, they are not unmixed blessings. A high " birth 

 rate " and a high percentage of " infant mortality " are concomitant necessities, 

 and economic or bionomic waste is thus entailed. Herein, to an extent which as 

 yet does not seem to be generally realised, migration plays a highly important 

 role. 



It would be difficult to find two localities equally favourable to the existence 

 of a given species of insect. Temperature, exposure, humidity, soil, etc., either 

 exert direct influence on its fortunes, or a more or less indirect but none the less vital 

 influence according as they favour or disfavour the growth of vegetation, and the 

 prevalence or absence of natural enemies, such as birds, etc. An insect not given 

 to migration must, if it be generally distributed, possess a potential reproductivity 

 sufficient to enable it to hold its own by mere force of numbers (by high birth 

 rate) in unfavourable localities, and this birth rate must equally be checked by 

 equally high " infant mortality " in the favourable localities through " facul- 

 tative " control. With an insect given to migration (more especially when this 

 migration is governed by a specialised instinct which lies dormant when condi- 

 tions are unfavourable to rapid increase, and the species is locally or seasonally 

 less abundant than usual), it is possible very largely to avoid this sacrifice of life 

 by the movement of superfluous individuals from particularly favourable to less 

 favourable localities. Disastrous superabundance is thus prevented on the one 

 hand, and on the other the superabundant individuals (which must be disposed of 

 in some manner) are so disposed of as to be of distinct utility* to the species, as 

 an entity, by permitting it to maintain itself in greater abundance in less 

 favourable localities. 



The broad general subject of " natural control " or, expressed in another way, 

 of the laws governing the natural equilibrium between co-existent species, 

 whether of insects or of the higher animals, of plants or of micro-organisms, has 

 received much attention of late from a wide variety of sources. In entomology 



* The writer has his own method for basing judgment on what is of utility or benefit to a 

 species (whether of insect or other animal), which it may not be out of place to outline. It is, 

 briefly, a presumption that the aim in life of every species is to multiply and populate the earth 

 —to exist permanently in the greatest numbers possible, over the widest territory possible. 

 Anything favourable, in its final analysis, to greater local abundance or wider geographical 

 dispersion is held to be advantageous— or vice versa. 



