8o THE SCIENCE OF LIFE. 



are first developed, then the characters revealing its 

 class." * Dr. Carpenter says : " The human embryo is 

 never comparable with a Fish, a Reptile, or a Bird, much 

 less with an Insect or a Mollusk. However close may 

 be the resemblance between the embryo of Man and the 

 embryo Fish, there is no real correspondence between 

 the embryo of Man and the completed Fish. Each germ 

 has a certain capacity of development peculiar to itself, 

 since like produces like" 



9. To attain a true knowledge of the order of creation, 

 or of the types of structure among organic forms, it is 

 necessary not only to consider internal construction and 

 relationships, and the process of embryonic development, 

 but also to trace the life-history of each, and especially 

 the metamorphoses to which they may be subject at 

 various periods. Among the lower Fungi there is a 

 kind of polymorphism (polys, many ; morpha, form) fre- 

 quent, by which several forms may be developed by 

 spores, or seeds, which have identically the same origin. 

 Few animals come forth from the egg in perfect condi- 

 tion. The embryonic Star- fish has a long body, with 

 six arms on a side, from one end of which the young 

 Star-fish is budded off. Soon the twelve-armed body 

 dies, and the young animal is of age. Most Insects un- 

 dergo complete change of form, a metamorphosis; i.e., ex- 

 hibit four distinct stages of existence egg, larva, pupa, 

 and imago. Among the vertebrates the most common 

 and most remarkable transformation is that of the Frog. 

 It is first, after hatching from the egg, a tadpole, with a 

 tail, but no legs, gills instead of lungs, a heart like that 



* Orton's " Comparative Zoology." 



