12 
fecund, and rapidly peoples and possesses the invaded territory. The 
peace-loving and more or less effeminate race occupying the soil disap- 
pears or is swallowed up by or merged into the more vigorous invading 
stock. In the course of years or centuries the latter in turn becomes 
fixed to the soil, enervated by increasing wealth and luxury, and falls an 
easy prey to the next similar invasion. In a sense it is true that a 
migratory race in some degree is of picked individuals, the weak and 
incapable being necessarily left behind. This, however, applies only 
to voluntary migrations. The transportation of insects attached to 
their host plants to new localities is not accompanied by the same 
natural selection, and furthermore, even in voluntary migrations such 
selection of picked individuals is not very thorough, and often the entire 
_ Stock is involved in the movement. 
The true explanation is undoubtedly in the mental or nervous stimu- 
lus born of very necessity and struggle and war for existence, which 
furnishes an energizing and bracing excitement. The same principle 
operates.in our Own case; we appreciate the renewed zest of a fresh 
line of work, the bracing effect, often temporary, of a new climate, the 
exhilaration from the reading of a new and interesting book or indulging 
in a new form of attractive sport. In the vastly greater and more 
important struggle for existence, this mental excitement, with its 
accompanying brace and stimulus, is proportionately increased, and 
explains very largely to my mind the unusual vigor and fecundity so 
often witnessed in the case of introduced plants and animals. 
This is, however, a temporary condition, lasting only through two or 
three or’ at most a small number of generations, and therefore in the 
case of insects frequently disappearing in one or two years, and rarely 
lasting for any considerable period. Very soon the new pest ranks 
with the old ones, and is occasionally abundant only, or often assumes 
‘avery secondary place. Note, in illustration, the severity of the horn fly 
for one season, or at most two, in each zone of territory represented by 
its spread inland, and many other similar cases which will occur to any 
of us, the most notables ones coming in my own experience being the 
eastward progress of the Colorado potato beetle and the purslane 
caterpillar. 
It is interesting in this connection to note briefly the explanations 
commonly accepted hitherto for the phenomena in question. These 
explanations are undoubtedly true to a degree, but seem to me entirely 
inadequate of themselves. I quote a paragraph from Mr. Lounsbury 
giving the view commonly held, not because he originated it, for it is 
very old, but because he has worded it recently and in brief form. He 
Says: 
The increased ravages of injurious insects in new countries result from the improved 
conditions under which they become placed. The relations which have existed 
perhaps for centuries between them and their food plants, their parasites and pre- 
daceous enemies, such as other insects and birds, are all suddenly broken. In their 
