104 TRUCK-FARMIHG AT THE SOUTH. 



dity of the grain-aphis was still greater. The wingless 

 females become mothers at three days old, and bear four 

 little ones every day, every one of which is a fertile fe- 

 male from birth; so that in twenty days the descendants 

 would exceed two millions. Only the next to the last 

 brood of the season consists of both male and female, 

 when the eggs are fertilized for every generation of the 

 ensuing season but the last, all the other broods being 

 born alive. No insect is more subject to being preyed 

 upon. In all nature a contest is going on for existence, 

 the weaker always succumbing. This is the struggle for 

 the " survival of the fittest," as Darwin terms it. 



If we sow our seeds too thick, we see some of the plants 

 overtopping and smothering the rest; we see in a growth 

 of young pine saplings that some of the trees are dying 

 out, in order that the more vigorous may have more room. 

 In this wise ordinance of the Creator, that only the 

 stronger individuals shall propagate their own species, 

 He has established a safeguard against deterioration; and 

 it seems that He has not only endowed the stronger with 

 greater powers to resist deleterious influences, but has 

 implanted in such lower orders of beings, which are fre- 

 quently the cause of disease and death, an inclination, a 

 selection, to infest victims least able to resist their in- 

 roads. The sleek, well-fed horse will not be subject to 

 itch; the well cultivated and vigorously growing orchard 

 is not apt to be attacked by bark- lice. 



The above is, at least, the only reasonable explanation 

 I am able to offer of the fact, often observed by cabbage 

 growers, that plants in luxuriant growth are rarely at- 

 tacked by leaf -lice, whereas a field that is backward in 

 growth, and feeble in health, will be overrun by them. 

 This is such a general observation, that Gregory says: 

 " Considering the circumstances under which the insect 

 appears, I hold that it is rather the product than the 

 cause of disease." 



