Xll INTRODUCTION. 



mous with living beings. There are, however, 

 some organisms of exceedingly simple structm'e, 

 which offer apparent exceptions to the universality 

 of this distinction. 



3. Chemical Constitution. — The body of every 

 living being is found to be composed of the essen- 

 tial elements, carbon, hydrogen, oxygen, and, 

 probably, also, nitrogen, to which other simple 

 substances may be superadded. No such uni- 

 formity can be predicated of the composition of 

 inorganic bodies. 



4. Mode of increase. — The increment of inor- 

 ganic bodies is effected by the addition of similar 

 particles to their exterior, while that of living 

 beings is due to the assimilation of nutrient 

 matters received into the substance of their bodies. 

 To such a mode of increase, the term groivth is 

 more properly applied. 



The organic world includes two great groups 

 of beings, plants and animals. These agree 

 with one another in the possession of the above- 

 mentioned characteristics, by which, as a whole, 

 they differ from inorganic bodies. They agree, 

 also, in their ultimate structural characters. They 

 have, moreover, a similar origin, since both alike 

 spring from germs, i.e. minute independent living 

 molecules, for the production of which parent 

 organisms are necessary. How then may they 

 be distinguished from one another ? 



