THE MICROSCOPE IN BIOLOGY. 127 



supposed that every animal passed through, in its devel- 

 opment, a series of stages in which it resembled the infe- 

 rior members of the animal scale, and systems of zoology 

 were proposed to be founded on this dream of embryology. 

 Careful research, however, has shown that larval changes 

 present many variations. In some the young exhibit the 

 conditions of adults of lower animals. Thus the Eolis, a 

 univalve shell fish, in its young state has all the charac- 

 teristics of a Pteropod, a free-swimming mollusk. Some- 

 times development is retrograde, and the adult is a de- 

 graded form as compared with the larva, thus setting at 

 nought all our theories, and teaching us that it is better 

 to observe than to imagine. 



12. Discrimination of Living Forms. We have seen, 

 section 6, that there are different kinds of living matter 

 endowed with different powers. We have also seen, sec- 

 tion 7, how varied are the forms of multiplication. Yet 

 when we come to discriminate between animal and vege- 

 table life, we find it exceedingly difficult, especially in 

 their more simple forms. Neither form, nor chemical 

 composition, nor structure, nor motive power, affords suf- 

 ficient grounds for discrimination. Yet when we consider 

 the functions of bioplasm in its varied forms, we may con- 

 veniently group all living beings in three great divisions, 

 viz.,/*m^', plants, and animals. 



The bioplasm of the plant finds its pabulum in merely 

 inorganic compounds, while that of the animal is prepared 

 for it, directly or indirectly, by the vegetable. The func- 

 tion of fungi appears to be the decomposition of the formed 

 material of plants and animals by the means of fermenta- 

 tion or putrefaction, since these latter processes are depen- 

 dent on the presence of fungi. Thus by bioplasm are the 

 structures of plants and animals reared from inorganic 

 materials, and by bioplasm are they broken down and 

 restored to the inanimate world. 



