Project Gutenberg #20417
The Outline of Science, Volume 1
J. Arthur Thomson
1922Thomson's broad popular-science survey of astronomy, biology, and Earth science, prepared section by section from Project Gutenberg HTML.
Project Gutenberg #20417 Public domain in the United States Cover source Local typographic cover created for MojiMori from public-domain source metadata
35
words at B2 or above
A1
A2
B1
B2
C1
C2
C2+
highlighted
at or above your level
Missing dictionary data: 3 unique words are not in the dictionary data yet. Amber marks show where they appear.
It sometimes happens that the inter-relation established between one living creature and another works in a retrograde direction. This is the case with many thoroughgoing internal parasites which have sunk into an easygoing kind of life, utterly dependent on their host for food, requiring no exertions, running[Pg 107] no risks, and receiving no spur to effort. Thus we see that evolution is not necessarily progressive; everything depends on the conditions in reference to which the living creatures have been evolved. When the conditions are too easygoing, the animal may be thoroughly well adapted to them—as a tapeworm certainly is—but it slips down the rungs of the ladder of evolution.
This is an interesting minor chapter in the story of evolution—the establishment of different kinds of parasites, casual and constant, temporary and lifelong, external hangers-on and internal unpaying boarders, those that live in the food-canal and depend on the host's food and those that inhabit the blood or the tissues and find their food there. It seems clear that ichneumon grubs and the like which hatch inside a caterpillar and eat it alive are not so much parasites as "beasts of prey" working from within.
But there are two sides to this minor chapter: there is the evolution of the parasite, and there is also the evolution of counteractive measures on the part of the host. Thus there is the maintenance of a bodyguard of wandering amœboid cells, which tackle the microbes invading the body and often succeed in overpowering and digesting them. Thus, again, there is the protective capacity the blood has of making antagonistic substances or "anti-bodies" which counteract poisons, including the poisons which the intruding parasites often make.