342 



GENERAL BIOLOGY 



In autumn the descendants of these will grow into an adult 

 form very like that shown in the first figure (fig. 192), and 

 will fly back to the witch hazel, and the young of these 

 developed upon the witch hazel will be wingless males and 

 females, all the other generations through the year having 

 consisted of females alone. There are other minor differ- 

 ences, none of the seven generations of the season being 

 exactly alike either in adult form or in developmental 

 stages; but the two forms shown in the figures are certainly 

 so different they would not be thought to be one and the 

 same species by any one who did not know their life history. 



Other cases of change of form with alternation of host are 

 well known ; probably they are more numerous than we now 

 realize, because of the great difficulty of recognizing the 

 identity of the different forms. The best known are perhaps 

 among animals the liver-fluke of the sheep (whose host ani- 

 mals are the sheep and the snail ; an account of it may be 

 found in almost any general text book of zoology) ; and 

 among plants, the wheat rust (whose host plants are wheat 

 and barberry ; an account of it may be found in almost any 

 text-book of botany). 



We will now leave these cases of heteromorphic adult 

 organisms, which though striking are rather rare and in- 

 consequential, and consider the far more common form- 

 changes that occur in the life time of single individuals. 



IV. METAMORPHOSIS. 



This is the name applied to changes of form undergone 

 after the close of embryonic Hfe — ordinarily, after hatching 

 from the egg. These changes may be inconsiderable, as 

 in the earthworm, or the leech (fig. 194), but in a number of 

 the higher groups of animals they are so great that the 

 young of many forms were originally described as inde- 

 pendent organisms, and given separate names. Thus 



