11 
conditions has brought about a balance in which the plant and its insect 
enemies exist together without notable danger to the host. The disas- 
trous effects to the host plant are, therefore, not due merely to the 
unusual increase and prolificness of the new enemy, but to the fact that 
the plant is confronted with an unfamiliar danger to which it for the 
time yields readily. The San Jose scale sometimes kills the plants 
which it infests, but the same plant, similarly infested with some old and 
familiar scale insect to which it has become wonted by long years of 
association, suffers comparatively little injury. Similarly, the Phyllox- 
era is almost kindly in its relations to native American grapes, while 
the unwonted European sorts soon succumb to its attacks. 
The normal powers of resistance to insect attack exhibited by healthy 
plants growing under favorable conditions is one of the most notable . 
phenomena of nature, and the workings of the same principle are seen 
in every other department of life. The explanation is doubtless both 
physical and chemical. The vigorous growth of the healthy plant may 
often smother or crush the delicate egg or young of the insect enemy, 
but the chief reason is unquestionably in the character of the juices 
and secretions of the weakened, stunted, or diseased plant, which 
render it more suited and especially attractive to its insect foes. 
From the other point of view, namely, that of the parasite or introduced 
plant or animal, there enters the very important effect of the new 
situation and new climatic and other conditions, the direct energizing 
-influence of which has, I believe, never been especially considered. I 
believe that the records fully demonstrate that the introduced plant or 
animal enjoys a very notable advantage from the great vital stimulus 
coming to it from its new climatic surroundings and conditions of life. 
Of necessity it is led to put forth unusual effort and to enter the strug- 
gle for existence with more than normal activity to overcome the dif- 
ficulties presented by new conditions, and as a rule this reacts to develop 
a more vigorous and a more fecund race. 
This principle is illustrated in the higher forms of life as well as in 
the lower. Our forefathers, coming to the New World, met hardship 
and adversity, and found it necessary to strive desperately for the very 
necessities of life. The intensity of this struggle is apparent to any- 
one who will look carefully into the early colonial history of this 
country as pictured by Bancroft, Parkman, Roosevelt, and Fiske. The 
very conditions, apparently unfavorable, of those early days, developed 
a race of stalwarts rarely equaled in later days; men and women of 
brain and brawn, who were possessed of a virility and force which 
extended well into old age and vastly in excess of that shown by the 
stock from which they sprang in the Old World. Families of twelve or 
more children were then the rule, where four or five are now considered 
large. 
The history of the world teaches nothing more plainly than that the 
migratory race is not only masterful and vigorous, but exceptionally 
