20 HISTORY OF THE HUMAN BODY 



condition is indicated in the diagram by such forms as 30 or 

 300 where the shortness of the line connecting the living form 

 with its ancestor indicates but little change from the earlier 

 condition. 



VI. As a given environment tends to exert a similar influ- 

 ence upon all of its occupants, members of quite distantly 

 related groups which become associated in the same environ- 

 ment often become so similarly influenced as to bear, super- 

 ficially, at least, a great resemblance to one another. This is 

 called " .analogical resemblance" and has been productive of 

 many mistakes in the attempt to clear up phylogenetic rela- 

 tionships. 



Many striking examples of this are found among verte- 

 brates. Thus a pelagic environment, as seen among the 

 extinct ichthyosaurs and the modern Cetacea, has changed 

 the fore-limbs into fin-like paddles, reduced the hind-limbs to 

 functionless rudiments, shortened the neck, and given head 

 and body a piscine form; limbless, attenuated forms occur 

 among fishes, amphibians and several groups of reptiles other 

 than snakes ; and a grazing habit produced in the herbivorous 

 reptilian group of the dinosaurs a close resemblance to the 

 large ungulate mammals of a later day. 



This law is illustrated in the diagram by the forms included 

 by the dotted line, which represents a given environment, in- 

 vaded by members of several groups. Here the descendants, 

 not only of related forms like 5 and 6, but those of quite distant 

 ancestors, as 6 and 30, have become similarly modified, until 

 they may resemble one another so closely as to deceive the 

 casual observer. Forms 20 and 33, representing totally dis- 

 tinct stocks, may thus bear so close a superficial resemblance 

 as to be popularly classed together under the same general 

 term.* 



* Thus whales and porpoises are vulgarly supposed to be fishes ; shrew- 

 moles, mice; and bats, birds. Salamanders are usually confused with 

 lizards; and certain blind and limbless lizards (Rhineura) which occur in 

 Florida and burrow in the earth, so closely resemble earth-worms as to 

 deceive at first glance a professional naturalist. 



