384 THE EA'OIiUTION THEOKY 



suited. Thus tho mainiiuils arc luug-broatihovs, and tlioir oxtrouiitios 

 are obviously adapted for locomotion on tho solid earth, yet several 

 groups have returned to aquatic life, as, for iustanee, tlie I'MUiily ol' 

 otters and the orders of seals and whales. Thus among insects which 

 are adapted for direct air-breathing, certain families and stages of 

 development have returned to aquatic life, and lia.\Hi dex'eloiied 

 breathing-tubes by means of which they can suck in air from tl\e 

 surface of the water into their tracheal system, or so-calle<l trat'luv>l 

 gills, into which the air from tlie M-ater ditt'usos. But tho luost con- 

 vincing proof of the organism's power of adaptatiitn is to be found 

 in the fact that the possibility of living parasitically witliin othei' 

 animals is taken advantage of in the fullest manner, and by the 

 most diverse groups, and that their bodies exliibit tho most marxoUous 

 and far-reaching adaptations to tho special conditions pre\'«iling 

 within the bodies of other animals. We have already roforrod to 

 the high degree reached by these adaptive changes, how the parasite 

 may depart entirely from the type of its familj^ or order, so that 

 its relationship is difficult to recognize. Not only have nmuerous 

 species of flat worms and round worms done this, but wo iind 

 numerous parasites among the great class of C/rustaceans : there 

 are some among spiders, insects, medusoids, and snails, and fchoro 

 are even isolated cases among lishes. 



If we consider the number of obstaeios that liave to bo o\'orcomo 

 in existence within other animals, and how difficult and how nmch 

 a matter of chance it must be even to roai^h to such a })lace a.s, for 

 instance, the intestine, the liver, the lungs, or even the brain or tho 

 blood of another animal, and when, on the other hand, we know how 

 exactly things are now regulated for e\'ery parasitic species so that 

 its existence is securctl notwithstanding its (Iepend(nico upon chance, 

 we must undoubtedly form a Iiigh estimate of tho jihisticity of the 

 forms of life and their adaptability. And this impression M'ill only 

 be strengthonod when we remember tlmt the ma.jorit_y of interna.1 

 parasites do not pass directly from one host to another, but do so oidy 

 through their descendants, and that these descendants, too, nnist 

 undergo the most far-roaching and often une.\pected adaptations in 

 relation to their distribution, their penetration into a new host, and 

 their migrations and change of foi-m witliin it, if the existence of the 

 species is to be secured. 



We are tempted to study these reflations more closely; but it is 

 now time to sum up, and we nnist no longer lose ourselves in wivdth 

 of detail. JVloi-eovor, the lifo-liistory of many parasites, and of the 

 tape-worm in particular, is widely known, and any one can easily fill 



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